I actually just posted this on another thread about brushing lol.
The vet I worked for was a big brushing advocate and here's what we taught our clients and what i did with my dogs:
First get them to enjoy the toothpaste (since most pet toothpastes are chicken or beef flavored, this is pretty easy) - don't worry about actually brushing the first few days - just put some of the toothpaste on your finger and let them lick it off followed by praise and a treat and end there. (I did it for about a week, until they saw the tube and came running for it, then I moved to step 2)
Next few days - put the toothpaste on your finger and rub the paste on their teeth followed by a treat. (in your case though, be carefull if she already snapped at you - wait till she's really likeing getting that toothpaste lol)
Once they're used to that and happy about it, start with the brush.
Mine love the CET chicken flavored toothpaste that my vet sells. they still don't really like getting their teeth brushed, but they love the paste enough to let me and they kind of chew on the brush like in the video posted so that works too
