With the tools I showed in the last post, these are the techniques I am using to cut the foam insulation board into the shapes and sizes I need.
As the saying goes, measure twice and cut once.
After everything is measured and the lines are marked I just follow the lines with the blade of the utility knife, no need to go all the way through the board.
This is a similar technique that is used to cut drywall (sheet rock), foam core board, and of course styrene sheet.
Then using the utility knife I scribe along the longer line and then snap it by bending.