Using a drywall router is a lot like routing wood or laminate with a guide bit. The drywall-cutting bit we use for electrical boxes has a “guide point”—a ring around the tip that won’t cut into the edge of the box, so it acts just like a guide collar [A]. The process is more or less the same whether you’ve traced the outline of the box or just marked the center.
To make a cutout with only a center mark to work with, turn the router on, then poke the bit through the drywall at the center mark and work toward the edge of the box [B]. When you hit the edge, pull the router back just enough to “jump” over the edge of the box, which moves the bit from inside to outside the box. From there, work counterclockwise, keeping the tip of the bit in contact with the box all the way around. Slow down at the corners to prevent over-shooting (something that gets easier with practice).
If you’ve used magnets or some other method to locate and trace the outline of the box, you can start the bit on the outside of the box by plunging through the drywall on or immediately next to your outline [C]. From there, press the tip toward the box to make sure you are, in fact, on the outside of the box, and work counterclockwise to make the cutout.
Either way, the result is a clean cutout that will be easily concealed by a standard coverplate [D].