In most cases, a simple tool such as a utility blade or X-Acto knife can cut your tool foam. These blades are fairly small, which allows you to cut small shapes without too much trouble. You can also use a heavy-duty foam hot knife for foam cutting. You can make clean, precise cuts with these knives.
Higher end cases like the Pelican use closed cell foam which should be cut out around the silhouette of the gun. Cheap cases use open cell foam which compresses easier and does not need to be cut. Open cell foam also holds moisture throughout the material just like a kitchen sponge.
Take scissors, a simple blade, cookie cutters, or even dental floss and cut through your foam. To make larger cuts, use electric tools. Secure the foam and use an electric knife, hot wire cutter, or foam saw to cut through thick or firm foam.
All you need is a damp cloth. Make sure your cloth is lint free and does not contain an excessive amount of water. Gently wipe the foam board or gator board from top to bottom. You do not need to use excessive pressure.
When gluing foam, apply adhesive to both surfaces evenly, and wait a few seconds for the glue to get tacky. Once you've properly aligned your cut foam, compress it together as firmly as possible. With foam's absorptive nature, this pressure works the glue deeper into each piece.