I started by, at the same time every night, just getting them used to me rubbing my finger lightly along their lip line, either side. Most cats like this any way. (if you are planning to use a brush, you might start this with the brush. I have always just used my finger)
While doing this, I was putting the c.e.t. toothpaste, poultry flavor on a paw for them to lick off, and get used to the taste. (they actually like it!)
This helps them associate the finger (or brush) rubbing along the mouth with the taste of the c.e.t.
Then, I started making the action more purposeful, putting them in the "holding position", which is: me on my knees, feet crossed behind me, cat between my knees facing out.
With cat in holding position, using a little more pressure which parts the lips, I'd be rubbing their teeth and gums instead of the lips, while they get used to me restraining them while doing it. After the action, again, putting a little paste on the paw.
Again, so now they are associating the action with the taste of the c.e.t.
From there, I put the c.e.t. toothpaste on my finger, hold them in position, and rub it into their teeth and gums.
This was a bit messy, so lately I've learned to use my other hand to kind of pry open the lips so I can make direct contact with the molars and gumline.
All this took time of course, but cats can get used to any sort of handling if it's done gradually and gently enough.
Oh,I forgot to mention..I always wash my hands before starting and between each cat.