Working with foam board: making sides / edges neat without any foam showing.
A 90 degree angle.

First, using a engineer square and a sharp cutting knife, practise cutting straight lines, using the excess strips cut off, cut 2 squares. On 1 square made a cut but not all the way through, just enough, so i could scrape off the foam, leaving the card attached. Glued the other square on. Now i have neat edge. (as so) Mine did go a bit dodgy, lucky it were a test!

Can stil see foam on the surrounding edges: to cover the surrounding edges, using cartridge paper cut some strips the same with as edge, then simply glued on, cutting angles where needed.