dont do it for her. let her do it. chances are she is not going to do it "right" or "well" but the more you let her be controlling of it the more able you will be to get her to do it later.
what we did with our son is we gave him the toothbrush, and he liked to eat that toddler stuff off of it, and we would sing the ABC's (or whatever song you want to sing) and we would stop singing if he took it out of his mouth. once in a while take a few SECONDS to hold her hand and brush her teeth, showing her the process. it also helps a LOT if you brush your teeth too, even if its just the toothbrush with no paste. thats probably best, because at this point you dont want her to have your toothpaste, or even want it :P
so give it a try. let her do it, never force it, some nights it just isnt going to happen. but doing it for her is going to equate toothbrushing with the being held down and fighting - not a good attitude to have about it! just try to guide her, and show her by your example of brushing and singing distracts enough that they might just brush a longer time than you expect!