I'm no expert since I only today bought some zipper by the foot and some sliders. I also bought a little 4 page thing that talks about zippers. But I did manage to get a slider on the zipper and it's not too hard.
You just separate a couple of inches on the end of the zipper (May already be separated), put one side of the zipper in the slider about 2/3, then the other side in and you just kind of play with it to try to get the slider on evenly. Once it's on, you just open the zipper up all the way (ahead of the zipper slider), and then close it back up with the zipper slider.
This page kind of goes over it:
Also, I might be wrong, maybe someone can correct me if I am, but I think if the zipper has not been installed yet, you can be off a couple of teeth on each side of the slider, in other words, not having each zipper half aligned, and then cut the ends afterwards.