Do you work mostly with speedlights? If so, congratulations. Gelling your lights is going to be quick, easy and cheap. The fresnel lens on your speedlight (where the light comes out) is only about 3-4 square inches. And that is all you are ever going to need to gel on a speedlight.
As far as how to attach the gel, you have several choices and all are simple. But there are also a few things to watch out for, unless you want your gel to be permanently heat-welded to your flash. Read more »