I'll admit it, I'm abusive to my pencil bags. When I decided to convert my old T-shirt into a pencil bag I realized that the thin T shirt material would need more reinforcement if it was going to last long in my possession.
This tutorial includes how to reinforce a single piece of T-shirt fabric and how to attach a non separating zipper to the bag.
Start off with an old T-shirt with a logo that you would like to save.
Take your zipper and compare it with the logo to estimate the size of your bag. I wanted mine a bit longer than the zipper bit so that my knitting needles could fit in the bag.
Cut out your logo into a square. making sure that the sides are as even as possible. For the reinforcing fabric, cut out a piece the same size as the T-Shirt fabric (I used duck cloth for my reinforcing fabric, which is really tough!)
Pin the two pieces of fabric together, with the logo side facing up.
With your iron set to steam, fold over the edges where you would like the zipper to attach and create a hem for the zipper to sew on to.
Cut off the uneven edges for a nice straight edge.
Position your zipper in the middle of the fabric, as even as possible to the parallel edges.
Gently fold the fabric up and pin it to the zipper, making sure that all corners are straight and even.
Carefully unzip it. If you used a zipper that pulls apart, simply pull it apart all the way and sew up the edges. If you have a zipper that does not pull apart, follow the following steps.
Starting from the unzipped end (or the end where the zipper head pulls up to) Sew the hem down to the zipper head, carefully going as far as you can to the bottom without either sewing to the other side or bumping into the zipper head.
Once you reach that stopping point, pull the bag out and pin one side of the hem to the bottom end of the zipper fabric.
This is the tricky part, pull the zipper down partways to prevent accidentally sewing over the opposite zipper edge, and sew down starting from the closed end down until you meet with the opposing sewn edge to complete the stitch. Repeat with the other side, being careful not to sew the other T-shirt side to the zipper.
Once you are done sewing the zipper, take a moment to step back to test the zipper and cut away any loose threads so that they won't get stuck in the zipper teeth.
Once you are satisfied with the sewn zipper, turn the bag inside out.
Flatten the bag with the zipper side up, and pin the corners so that they are flat and even together.
Sew down the corners to the zipper fabric.
Both ends should be sealed off towards the corners, leaving you with a solid fabric tube.
Pin the T-shirt fabric to the reinforcing fabric to ensure that the T-Shirt fabric stays flat to the edges.
Sew straight down the ends.
Your bag should look like this. The duck cloth was too stiff for sewing fancy edges so I left it as is.
Turn the bag inside out, zip it up and it is ready to store your treasures!