Brush
Let your dog sniff the toothbrush and lick a bit of toothpaste first so they get used to the taste before anything touches their teeth. Once they're comfortable, gently lift the lip and brush in small circles along the outside of the teeth, paying extra attention to the gum line where plaque builds up fastest. Daily is ideal, but consistency matters more than perfection.
If your dog isn't sure about it
That's normal, especially early on. Stop before they get stressed, offer a treat, and try again tomorrow for a slightly longer stretch. Most dogs come around within a week or two of short, low-pressure sessions, it's not a race.