I had lost my pencil pouch so I decided to make a new one. I don't have much fabric so I came up with the idea of using denim from a old pair of jeans.
If there's anything that's unclear for you (Hopefully there aren't many complicated explanations, but this is my first how-to!), feel free to comment. ;)
Take one denim rectangle and one rectangle of your lining fabric. First, put the denim on your work area so that the outside part (the darker side) faces up. Then grab your zipper and place it on top of the denim so that the two top edges match up. The flat bottom of the zipper head faces up. Next, take your lining rectangle and place it on top of your denim and zipper so that the non-patterned side faces up. You should end up with a sandwich with your zipper in the middle and the two patterned sides of the fabric faces each other. When you open your "sandwich", it should look like the picture.
Now sew all the layers together. Make sure that you're sewing as close to the zipper as you want (you don't have to sew just on the edge of the sandwich).
Repeat step 3 and 4, sewing the last two rectangles onto the other side of the zipper. Sandwich:
Denim - face up
Zipper - lined up to top edge of denim, flat bottom of the zipper head faces up
Lining fabric - design faces down
Make sure the end result looks like the picture (the lining fabric is under the denim).
Unzip your zipper. Then line up the fabric rectangles like in the picture (the denim rectangles face each other; ditto for the lining fabric). Sew on the black and white lines in the picture. Make sure that you leave a hole for the lining fabric (indicated by the red circle). You need the hole for turning whole thing inside out.