Dear Phae,
It all depends upon what type of zipper you're putting in. A slot zipper is never used in good garments; you should use the lapped zipper application, and on very good clothing, you should hand-prick the last step.
Look at the pictures that come with the zipper, and read those directions. This is the best, and easiest way to install a lap zipper. If you need more help than that, just ask.
The trick is to machine baste the opening before ever starting with the zipper. I leave a finger-wide space at the bottom of the basting, to act as a guide. When you have practiced this a few times, it will take you less than five minutes to install a perfect zipper.
A fly zipper is not basted, but the seam allowances are pressed in place before starting with the zipper. I tape the fly pattern piece to the front pant to eliminate several layers of seams. I press the underlap 1/4 inch into the seam allowance, and the overlap right on the seam allowance. I sew the underlap, from the top down, and back stitch. If a zipper guard is to be used, I topstitch it at this point, right over the original line of stitching.
I then pin the overlap in place, right next to the fold. Turn the piece to the wrong side, and sew the other side of the zipper right next to the teeth, but only to the seam allowance, not the outside. Then, go back to the outside and do the "fly" stitching. This zipper, too, can be applied in no time at all. Even novices can do this without basting or any extra steps.
Teri