OK, here goes...

You will need:
* A t-shirt
* Lining fabric: must NOT be stretchy since it's basically holding all the weight of anything put in the bag
* Thread the same colour as the shirt
* lots and lots of pins
* Fabric for the handles of the bag (or if it's a big shirt you can use some offcuts to make the handles)
* Seam ripper/unpicker
* Sewing machine/needle
* Scissors (der, I know, but since I'm listing every other stupid detail)

Step one: unpick the sleeves from the shirt. This will take anywhere between 2 minutes and 2 hours depending on the quality of the shirt. Unpicking gives you the most amount of material to work with, but if it's a big shirt you can probably get away with just cutting the seams out.
Do not throw out the sleeves once unpicked!






























Step Two: sew across the bottom of the shirt (above the hem, cut that bit off so it isn't bulky on the inside of the bag), and cut straight across the top (just below the neckline). Because I'm mainly using my own shirts and they're a bit larger than I would like for a bag, I sometimes have been taking in the sides or shortening the bottom. One thing I've learned is, it always looks smaller than you think. Measure your shirt against a bag you like the size of and adjust accordingly, because a "medium" ladies' shirt looks like it'll make a medium-sized bag but it actually makes a ridiculously massive bag.
ONLY sew up the straight sides of the shirt, not the sleeve hole! Leave the sleeve hole curve unsewed.

Step Three (may be done out of order if you want to be difficult): make the handles.
I generally buy a bit of drill cotton in the same/similar colour to make the handles out of because it's cheap and plain but pretty sturdy. However, I know how hard fabric shops are to come by in London so if it's a long or big shirt and you cut enough off, you can use that to make the handles instead. Use the same instructions, but after you've finished, do a few more rows of stitching along it to stop it from stretching (zig zag is best).
Cut a long strip of your fabric of choice, about 2 and a half times wider than you want the straps to be:















Iron the strip in half (right sides together, if you can tell which the "right" side is. I can't tell on drill, but t-shirt fabric usually has a nice side and a rubbish side):















Sew along the whole length. I generally leave about half a centimetre of seam allowance but it doesn't matter than much here, just do what you need to do to get the straps the right width.




















Turn the whole thing inside out (way easier with t-shirt fabric than non-stretchy drill).




















Iron the whole thing flat. I tend to do it so the seam is along one side, but other people like to do it so the seam is in the middle of one side (if you know what I mean. Doesn't really matter in my experience).















Step Four: Prepare your lining.
The way I like to do this is fold the lining in half, and use the fold as the bottom of the bag (it's stronger if it's just fabric rather than a row of stitching that might get stressed and come undone). So fold your fabric in half to the height of the bag (with about 2 or 3 cm extra on top .... trust me, it doesn't make sense, but you often need it!) and lay your shirt on top of it:




















(ignore the chalk marks in the picture. They were trace marks I drew onto the shirt to demonstrate the size of another bag I have that I like). Allow seam allowance on each side (generally about half a centimetre each side) AND a little bit more. Because your shirt is stretchy, if anything is going to be bigger, it should be the (non-stretchy) lining. You can always stretch out the shirt to make it fit the lining, but if the lining is too small it will not work. In general I would probably add about 1 or 2 centimetres.

Strep Five: Sew up your lining. Like your shirt, you want the two sides and the bottom sewn across, and the top completely open. EXCEPT you need to leave a GAP in one side so you can turn everything inside out later (again, trust me at this point, but this is the MOST ESSENTIAL STEP in the WHOLE THING. I know this because I've made that mistake before).


















It's not so clear in the above photo but this is the lining of my bag folded in half. So on the top you see one side of the bag where the side seam is sewed from top to bottom without stopping.
Below you see the side seam which is sewed up about 3 or 4 cm from the bottom, then I leave a gap unsewed (about 4-5cm is probably enough unless you're using really thick fabric, then allow more), then the rest is sewed up. My rapidly-Sharpie-drawn diagram may or may not help illustrate this point.
Turn the whole thing inside out (so that the "right" side of the fabric is facing out, and the seams and threads and mess are on the inside.

Step Six (optional): If you want inside pockets (wish I'd thought of this when I made your original bag, Di! Oh well, it was a prototype) then use the sleeves you've previously removed. Cut the seam open (the seam that would normally go from the armpit to the underside of the cuff). Lay it out flat and cut it to roughly the size you want the pocket to be, plus 0.5-1cm extra:















Iron the sides and bottom of the fabric under so that the pre-made hem of the sleeve is the nice, neat top part of the pocket: (this photo is showing what the pocket will look like from the inside (of the shirt).




















Pin this pocket to the lining where you want it and how you want it to look:


(ie. the bits that you've folded under should be touching the "right" side of the lining, and pinned down. I generally put it so that the top of the pocket roughly lines up with the bottom of the sleeve hole.
This may sound obvious, but only pin it to ONE side of the fabric! You can make pockets on both sides of you like but I generally only bother to put in one)





Then sew the pocket(s) into place along the two sides and the bottom. Be careful not to sew the two sides of your lining to each other (again, I know this because I made that mistake once).

Step Seven: I'm going to tell you right now, this is the step where you will have the most trouble, and this is also the step that will feel the least productive, but it's worth spending the time to get it right. If you're using a lining fabric that has a geometric pattern (gingham is an excellent grid to work with! Polka dot fabrics are quite good also) then USE IT to make your bag even. If not, do your best and be comforted by the fact that it's a stretchy bag that is MEANT to look home-made so if it's a bit wonky it won't matter. Just give it a good visual once-over before you do any sewing, to make sure it looks reasonably even.

You're going to pin the whole thing together now.

To start with, you need the shirt to be WRONG side out (design is on the inside), and the lining to be RIGHT side out (so pocket is visible, if you put one in). Put the lining INSIDE the shirt:




















At this point, I find it helpful to pin the corners of the lining and shirt together roughly, just to keep the whole thing in shape. You just push though the lining with a finger, and once the corners "fit together" put a pin through all four layers of fabric. It doesn't have to be exact, it doesn't have to be neat, it just has to stop you from making any really massive mistakes later!

Once your corners are in place, pin the side seams of the lining to the side seams of the shirt. Remember that the shirt fabric is stretchy, so if it doubt, always pull a little harder at the shirt and pin it a little further/higher than it sits naturally. It's hard to explain how I decide where the side seams will be sewn together, all I can suggest is, hold the seams together at the bottom corner, then move your other hand along the side seams and when you get to the top, pull the shirt about half a centimetre further up the lining and pin it there.















Pin the seam allowances OPEN (on the shirt and the lining. It'll make the sides of your bag sit much more nicely).

Now, pin the straps in BETWEEN the lining the shirt (only pin to the shirt part at this point. You still need to be able to manoeuvre it around the lining). You want to have the rough ends of the straps poking out the top, and the actual strap sitting "inside" the side of the bag:


You should be able to see from the photo that the edge of the strap is NOT flush with the edge of the shirt hole. It's pinned away from the edge, by however much seam allowance you have (again, usually about half a cm). This is IMPORTANT! The actual distance itself isn't that important, because as long as you don't need extreme symmetry, you can adjust a little bit as you sew, but the main thing is that you DO leave a seam allowance there.










Next is the hard bit: pick a sleeve hole, and pick a side of the shirt. Pin the shirt to the lining:

This picture is of the sleeve hole when I'm JUST about to sew. Don't do the WHOLE sleeve hole at one time. Just pin between the side seam and the strap on one side, then do it the same on the other strap/side. Follow the curve of the sleeve hole. Until you've tried this yourself you won't be able to comprehend how many different ways there are to do this - you can make the curve of the sleeve be very vertical or very wide and open as long as it sits flat.



Then lay the whole thing flat: do you have roughly the same amount of lining and shirt fabric sitting between the two straps on the side you've pinned? If you do, or you have just a little more lining than shirt, then you've done well, pin the top opening of the shirt to the lining (remember you can always stretch the shirt fabric to fit the lining). If you have way too much shirt fabric, you need to re-pin the sleeve holes to the lining, but make a more narrow curve (like the one in the picture above.). I know it all sounds quite complicated, but trust me that as soon as you have the thing in front of you it will make much more sense.

Fiddle around with it until it sits nicely. You should be able to look at it/feel it and understand right now, but if not, call me, I'll talk you thought it, I've been there. The first bag that I made (Di!) took several hours of me and my mum sitting at the dining room table staring at it and having a conversation that would make no sense to anyone else:
"But what if we ...."
"Oh, and then turned the ...."
"Or.... or, sewed the sleeve bit around ...."
(insert non-word vocalisations and gestures)
(more gestures)
"What time is your plane in the morning?"
"We have PLENTY of time!"
"Well then stop talking about it and start sewing!"
(one or both of us makes that noise that you (and by you, I mean me and my mum) make when you have pins in your mouth and are concentrating)
"Yes! And then that should ..."
"Move your one over one (gingham) square to the centre"
(Just be grateful this doesn't require me to be standing on the table wearing high heels while my dress hem gets pinned. That dialogue is similar but involves way more "this lampshade needs dusting")

If you have survived this stage, you should have a bag-within-a-bag that has the opening held together (neatly) with pins.

Step Eight: Sew the pinned opening to the bag all the way around. Go slowly around the sleeve holes to keep it even, make sure you turn the corners neatly, and remember that when you get to the straps, you want to seam around the sleeve hole to just barely touch the strap ONLY when you reach the corner. Again, hard to explain (I tried to take photos, but black sewing on black was NOT helpful). Around the sleeve hole, you want the seam to have nothing to do with the strap. But as you approach the top opening of the bag, head the seam towards the strap so you hit the corner between the seam allowance and the strap:




















Step Nine: Congrats! All the hard work is done! At this point, you need to pull the whole bag inside out though that hole you left back in Step Five.
At this point, your bag will basically look pretty good:




















What I tend to do from here is then do a line of sewing around the opening of the bag (from the outside) to give it a bit more strength.
Then you need to sew up that hole you left in the lining (ladder stitch).
You can also do a bit of reinforcement of the handles if you like, by sewing an X in a box around the handle:




















Step Ten: By now you should probably have your own rather special t-shirt bag good to go. Fill it up with all sorts of crap (seriously, these bags are enormous, you can fit way more than you think in there) and head on out. Show it off!