There are another option called "Dental Wipes" . They are disposable clothes soaked in toothpaste/cleaner. They come in a plastic container and come out like kleenex. One wipe is way more than enough to do.
The best way to do this is to have the animal on your lap or next to you. Have the head AWAY from your body!!! Do NOT have them facing you when doing this. It may not work and you could get bitten! You place the wrap around your finger like your cleaning a spot out. Then gently place it in their mouths gently rubbing the gums. You may want to just rub the gums for the first few tries if too much of a struggle. You can then move to the teeth with small circular motions. With cats, they tend to think your "nursing" them if you rub near their mouths. So it may be easier to just learn to rub them gently by their mouths a few times to get things "started".
I've used this method and thought the instructions on the box was crazy . I was like NO WAY!!! However, after cleaning the teeth of an old dog not familiar with me and never had dental care before, the method did work! It's kind of "gross" until you get used to it. Expect the results to take some time and the tarter not to come off immediately. Just work on the plaque buildup first.
I've tried the "toothbrush" and the "Finger brush" method all with limited success. One dog responds well with the toothbrush but the other doesn't. The cat forget about it! I use the wipes on her when she's "tolerable". I don't feed her any treats and only give her hard food so her teeth stay pretty healthy. Softer foods tend to cause more buildup and decay.