Clipping of overgrown teeth can be done with some types of nail-clipper or with a pair of small-beaked wire-cutters: whatever clipper you use must be small enough to fit round the teeth and still be able to see what you're doing, and sharp and powerful enough to cut the tooth rather than break it. If in doubt as to how long or short to cut the teeth, look in another rat's mouth for comparison and err slightly on the side of length - and be careful not to nick the tongue. The individual teeth are wider from front to back than from side to side, so if you are trimming one tooth on its own, rather than a pair together, it is usually more efficient to point beaked clippers into the mouth rather than across it, so the blades are pressing against the long sides of the tooth rather than its short front and back.
If you do cut a tooth too short - or split it - and make it bleed this will hurt, but does not seem to do so as much as a broken claw. I suppose that any animal that can chew through sheet metal can't afford to have very sensitive teeth.
The rate at which overgrowth occurs varies from case to case, but on a rough average you can expect to have to cut the affected tooth or teeth back about every ten to fourteen days.
I have never seen or heard of a case of overgrowth of the cheek teeth in a rat, but if it should occur then, as with rabbits, it would probably have to be clipped back under general anaesthetic owing to the extreme narrowness of the mouth, the unco-operativeness of the patient and the way folds of lip tuck in behind the incisors and conceal the back teeth from view.