First thing cut two long narrow pieces of cardboard that are the maximum length and width that will fit in your car. Mark these with a 'P' for pattern and keep them. Go for the biggest pieces first. Cut boxes flat by opening the top and bottom and cutting the corner edge that had the glued overlap. Use the patterns to rough cut the boxes into pieces that are as large as possible but will fit in your car.
Safe Cutting Cut the cardboard with a utility knife.
Always work cutting away from your body. Never work in such a way that if you slip you will cut yourself. If the boxes have large metal staples in them these must be removed with care before doing any cutting. Watch out for them even in flat boxes. They can cut you badly.