Trim the Edges
For foamcore or Gatorboard that is only 1/4-inch thick a box cutter will work fine. For thicker board use a heavier utility knife.
Turn the piece over on a cutting surface. Lay your ruler or straight edge parallel to the edge of the artwork, overlapping it just a little so no white border remains.
Pierce the surface of the board somewhere outside the image and then cut across the image in one clean stroke. Try to cut all the way through the board, keeping the blade completely perpendicular to the surface. Cut on a cutting surface or a piece of scrap mat board.
If you didn't manage to cut all the way through, take the blade out of the cutter and very carefully cut through the back by hand.
Smooth any rough edges with an emery board.