DIY Photography with Larry Becker: Gels
The only question left now is how to mount the gels on your flash unit. There are a quite a few options, but here are my favorite two.
Option One: Velcro
If you already have Velcro strips on your flash heads (and I do), then placing a circle of Velcro on rectangular strips of the gel will allow you to attach them easily. Also, if you put the Velcro “hook” side on one side of your gel rectangles and the “loop” Velcro on the other side, it makes it easier to stack them.