Building on a paw-touch behavior to teach a raised paw against your hand — a natural next step once your dog understands paw shaking.
High five is easiest to teach after your dog already offers a paw on request, since it builds on the same lifted-paw motion — just held higher and against your palm instead of resting in your hand.
If your dog doesn't have a paw behavior, gently touch the back of one front leg near the paw — many dogs lift a paw in response. Mark and treat any paw lift, even a small one, and build up from there over several sessions.
Once your dog reliably offers a paw into your open hand held low, gradually raise your hand a little higher each session. Keep the hand target flat and stable — a moving hand is harder for your dog to aim for.
Say "high five" just as you present your raised hand. Repeating this consistently over several sessions builds the association between the word and the raised-paw motion, separate from the hand position alone.
Practice with your other hand, different angles, and eventually other people's hands. A dog who only responds to one exact hand position hasn't fully generalized the trick yet — a bit of variety in practice fixes that.
This one often takes a bit longer than sit or down since it requires more precision from your dog. Short, frequent sessions work better than long ones — a couple of minutes, a few times a day, tends to get there faster than one long session.