I bought a small doggie toothbrush and some doggie toothpaste ... then I put them in a football hold and just went for it
They hated it !!
They squirmed and huffed and puffed about it but I just kept taking breaks and eventually after about the third/fourth time or so they would let me brush atleast a minute or so. Afterwards they get a treat.
I still don't get the backteeth very well so we use petzlife gel and that seems to be working for us.