Score the front (get a nice line on the paper side) with a drywall square and knife. Break from the back then cut along the broken fold line with knife. Use a rasp to tidy up any messy gypsum bumps.
Don’t put screws in too far or you’ll break the paper
Use corner bead on outside corners
Fold the crease in paper tape for inside corners then lay it on a bed of joint compound and press in with a corner trowel