All you need is two t-shirts (and cutting/sewing supplies).
Step 1: Cut out rectangles
The dimensions between the two shirts' rectangles should be the same on three sides, but the rectangles from one of the shirts should be a little taller (my gray one was about an inch taller).
The taller fabric, which I will henceforth refer to as the gray fabric, will be the outside for the drawstring pouch.
The shorter fabric, which I will henceforth refer to as the red fabric, will be the outside for the backpack.
Step 2: Cut strings
You will be turning these into your strings/backpack straps.
Divide each color type in half- the six cut stripes below all belong to one strap.
Cut the loops on one side to make them long strips.
Sew the ends together such that you'll end up with one red and two gray equally really long strips per strap.
Sew across the ends of the braids; cut loose ends.
Step 3: Sew rectangles
BUT WAIT! Leave a gap on each one:
Red fabric: leave a little (1"?) gap at the bottom corners of the bag.
Gray fabric: Remember how this was taller than the red one? Leave the bit that's taller unsewn at the top of the bag.
Step 4: Put it together
Roll the unsewn part of the gray bag over the top.
Sew along both sides, but leave both of the ends open (see pictures).
Step 5: Add drawstring
Pull it through one tube, then turn it right around and pull it back through the other tube, so it ends up on the same side it started on.
Repeat with the other string, but from the other side.
Step 6: Tuck in ends
Now, this bit is fiddly, but it's also clever, so follow along:
Push the corner of the red bag, with the strap ends tucked into it, up through the lining to come out through the tiny hole you left in the top (see the last picture).
Pull that corner out of the bag. It should be inside out, with the strap ends poking through. (see the second picture).
Sew in the gap that you didn't sew before (back in step 3), completing the seam and sealing the ends of the straps into it.
Push the corner back through the hole and tuck everything back into place again.
Was that confusing? Leave a comment and I'll see what I can do to help.
Repeat with the other side.
When you're done, it should look like the third picture.