(This looks complicated in words but if you follow the pictures, it really is not. What I wrote should just clarify things if you have questions.)
1. Decide where you want your pocket placed on your main piece of fabric. Place a pocket piece on top of where you want your pocket to go. Make sure you give yourself at least two inches of pocket fabric above where you want your finished pocket and at least an inch or more on the sides. *Make sure the pocket piece is on top of the main fabric right side up. **It is ideal if one of your pocket pieces matches your main fabric, but it’s not necessary (just looks cleaner and hides any small mistakes). If you have one that matches, use it as the first pocket piece.
2. Using your ruler and sewing marker, draw a rectangle 1/2 inch tall and as long as you want it to be. Make sure your zipper is long enough or longer than your rectangle. You also want to draw a triangle at each end with lines at about 45 degrees to the corners, a line connecting them, and a line marking the middle of the rectangle.
3. Place a pin at the middle line. Also place pins around the rectangle to keep everything from shifting.
4. Sew exactly on the rectangle you drew. Don’t sew on the lines you made inside the rectangle, just on the rectangle. With your scissors cut the lines on the inside (except for the middle marker line). Make sure you snip the lines to the corners as close to the stitches as you can without snipping through the sewn lines. The closer you get to the corners and sewn lines, the cleaner your welt will look (which I’ve learned through trial and error).
5. Push the pocket piece through the middle hole you just created.
6. Press generously with a hot iron to get it smooth and even. This is how it should look from the wrong side.
7. Place your zipper over the pocket opening. Flip your fabric over so that the main fabric is right side up again.
8. Pin the zipper in place. I like to pin in the direction I’m going to sew with the pin heads facing toward me, so I can pull them out as I go. I start by the zipper pull and pin all the way around.
9. Check your work on the back before sewing, to make sure everything is in place.
10. Top stitch around the zipper very close to the edge. Sew slow! Take your time. Start by the zipper pull and sew around pulling the pins out as you go. You will need to pull the zipper pull open a little and squish the zipper teeth together as you sew that first side. Be sure to sew forward and back when you start and stop.