Starting gradually is key. That, and using poultry-flavored toothpaste.
For my pups, I started by just putting a bit of toothpaste on the brush and letting them lick it off. Over the course of a few days, I gradually start just brushing the easily reached teeth. Once they seem comfortable with that, over the next few days, I become increasingly thorough, until I'm getting all teeth scrubbed. By this time it's routine, and they all line up to get their teeth brushed. They seem to like the sensation, and will sort of "air-chew" while I brush.
As for the exact how, I have them stand up, and I straddle their backs, leaning over their heads. This allows me to easily hook the corner of their mouth with a finger and pull the lips back with one hand, while brushing with the other so I can reach all those tiny teeth waaaay in the back (you know, the ones that always seem to decay first ).