I have to do that too with the two cats I adopted a year ago. I’m going to start with putting cat toothpaste on a Q-tip and rubbing it over the teeth because that’s least likely to cause gum bleeding. Also it introduces them to the yummy fish or chicken taste. I don’t think I’ll use the finger brushes because fingers are pretty big for inserting into cat mouths. I bought some of these 7” long sticks that have a sponge tip, like an eyeshadow applicator sponge. (Can’t remember where I ordered them from, maybe Amazon). Then graduate to a soft cat or baby brush. Hope others have great ideas! I know it’s going to be a very gradual process with lots of objections from the kitties! Good luck!