Two zippers of the same length (I ripped these off of an old binder I wasn't using)
A sewing machine
Placing the Zipper
Mark the center of the pant leg and draw a line from that mark that measures the same length of your zipper. Measure the width of the zipper and draw a line of that same length perpendicular to the top of your previous line.
Before you sew fold the rough edge of the jean under and pin the zipper in place so that you can only see the teeth of the zipper.
Clean It Up
Cut off all the loose threads. The zippers I used didn't have a metal piece at the end to keep it from falling off, so I'm going to take a paper clip and bend it around the bottom, maybe an inch from where the jean hits the floor, to keep it from falling off. I'm putting it an inch from the floor to keep it from wearing down. Enjoy!
If you had any problems understand this or any tips on how I could have done this better, please let me know.
Below is a crappy picture of what the cutting and sewing should look like. The black is where you should cut and the pink is where you should sew.