I taught Rylie to think it was the most awesome time of the day! She literally comes running when I take the toothpaste out.
I started out with letting her lick the toothpaste off my finger. That's it and praising her for it. Then when we were relaxed in cuddle time, I got her used to me touching her face and then lips and then mouth with my hands (also good for if they ever get something in their mouths you don't want them to have). Lots of praise!
Then I got her used to the toothbrush by putting toothpaste on it and letting her lick it off there. Every time the toothbrush came out and she came to it and was interacting with it, she got a treat and praise! Eventually, we got up to her letting me open her mouth and brush it. I started with just lifting the lips and doing front/sides only and then treats/praise.
Now she lets me just open her mouth and go to town. Some days she's less compliant than others (I think it has to do with the fact she's getting teeth in and losing others and her gums probably hurt...), but she's normally very good. She always gets a treat after grooming/teeth time!!!
I hope this is helpful. It worked for us. Good luck!
Maybe others have some good tips too!