Putting a zipper into a skirt is a little different than putting a zipper into a jacket in that the jacket zipper comes in two section and the skirt zipper is all in one.
Begin by laying out your skirt on a flat surface. Place the zipper (in this case a 12" dress zipper from my local fabric and craft store) along side the skirt's edge. Line up to the top of the zipper with the top of the skirt. Place a pin where the zipper ends.
This shows you how long the last seam in your skirt will be.
So place the two sides together, with needle and yarn, sew from this point (where you placed the pin) down to the skirt's bottom edge.
With the seam sewn, lay the skirt back onto the flat surface. Pin the zipper to each side of the skirt's opening.
Make sure when you pin the zipper that the edge of the skirt and the zipper's teeth are not close enough to touch--this will cause the zipper to get caught in the yarn of the skirt when opening and closing it.