Drywall is one of the easiest building materials to install, partly because it allows for minor errors. Most professionals measure and cut to the nearest 1⁄8", and it's perfectly acceptable to trim off a little extra from a panel to make it easier to get into a tight space. The exceptions to this are cutouts for electrical boxes and recessed light fixtures, which must be accurate, because the cover plates usually hide less than you think they will.
Make sure your utility knife is sharp. A sharp blade ensures clean, accurate cuts that slice through the face paper and score the gypsum core in one pass. A dull blade can slip from the cutting line to snag and rip the face paper and is more likely to cause injury.
With a sharp utility knife, you can make cuts from either side of panels. But when using drywall and keyhole saws, make all cuts from the front side to prevent tearing the face paper. For projects that require a number of cutouts, use a spiral saw. This tool makes short work of large openings and electrical boxes; though it generates a lot of dust, so make sure to wear a dust mask. Inexpensive spiral saws are available at home centers, or you can use a standard router outfitted with a piloted drywall bit.
