Fly-fronts are intimidating to me because of the topstitching. I just finished a pair of Palmer-Pletsch pants and the instructions had 3 different techniques for marking the topstitching. While reading my March 05 issue of Sew News I discovered a fourth technique.
The package of a basic Coats and Clark zipper has a curved flap on the back which can serve as a fly-front zipper guide. Just position it in place (using double-sided tape if necessary) and stitch next to the cardboard.
This use isn't mentioned anywhere on the package. And to think I've just been throwing those away then struggling with the topstitching.