It is based on the idea found here: https://www.instructables.com/id/FASTEST-RECYCLED-T-SHIRT-TOTE-BAG/#step, with a few little improvements of my own - which just means that it takes a little longer :)
Step 1: What You Need...
Step 2: Step 1: Preparing the 'handles'
Now fold the 2cm side part of the fabric over and pin it to the big part of the shirt. Try to pin the two layers as flat together as possible so there are no bumps when you sew it later. Make sure you fold it on the seam so it forms a nice, neat edge in the end.
Step 3: Step 2: Sewing Your 'handles'
After sewing the hem flat, trim the excess fabric off so that it won't start fraying and cause lots of funny threads when you use or wash it. Be careful not to cut too close to the stitches.
Step 4: Step 3: Cutting the Opening
After cutting the edging off, you will need to even the back and the front pieces out. You can do this by cutting along the edge that forms the front part. This will leave you with a crescent shaped piece, which is handy to add as a little pocket which can be sewn on the inside of the bag later on.
Step 5: Step 4: Sewing the Hem for the Opening
When you get to the seams of the shoulders, make sure you fold it is the same direction that you sewed it down when sewing the sleeve's hem.
Step 6: Step 5: Optional Pocket Time...
Step 5: If you want to add a pocket inside, grab that moon shaped piece of fabric. If you are like me and don't have an overlocker, simply sew a zig-zag stitch around the edge of the entire shape to prevent fraying. Once that is done, pin it the the back side of the bag ( make sure your 'bag' is still inside out before you pin, and make sure you only pin it to the one side!)
Now sew the pocket onto the one side, you can use a straight stitch as it will show up less on the finished product or you can use the same zig-zag stitch if you are using the contrasting thread colour so that it matches the rest of the stitches if you prefer.
Step 7: Step 6: Sewing the Bottom
Sew the seam first with a straight stitch and then with a zig-zag stitch for re-inforcing. I like to give me bags rounded corners, so if you want that, then just start a little higher up and sew a curved corner like in the image below.
If you need to, trim the seams before you turn your bag the right way out. Also remember to trim all your long threads left from sewing.
Step 8: Step 7: Final Step
After that, your bag is complete and ready to use instantly!