Start by cutting the tape, through the teeth and all, to the size you want. I use kitchen shears so I don't damage my good sewing scissors.
From the top edge of the tape cut into two or three of the teeth.
Sit the zipper slide at top...
Slightly push the slide onto the teeth.....
And gently pull at each side of the tape until the pull glides on the teeth.
At this point you want to secure the ends so the pull doesn't slide right off the tape when it opens and closes. I like to use a matching thread and just take a few stitches through the tape and over the teeth on each side of the top to give an end point for the slide.
Do the same at the bottom, just going across both sides of teeth and through the tape. Take several stitches so it is secure.
You now have a custom-sized zipper!