There are many good articles online about how to brush cats teeth. For me, I took the following steps:
1. Touch/massage my cats gum with my fingers frequently
2. Let them smell/lick the toothpaste from my finger
3. Rub the toothpaste on their teeth with my finger
4. Brush their teeth with toothbrush. At the beginning the session is short, like a few strokes, then I can do longer. I also give them a treat before and one after brushing.
The whole process took about 10 days for me. It might take longer/shorter on your cat. Once both you and the cat are used to it, its really easy. I brush my cats teeth every day, which takes less than 2 minutes per day