A long beard with short sides does not work. You have to trim or fade the beard into it or something, you can't go from a number 1 buzzed sides to thick beard instantly.

Undercuts are cool as long as there's some length on the top. Getting an undercut and then leaving/cutting the top very short defeats the purpose, it's called an undercut for a reason. The shaved part has to be under something.

Just tips from someone who used to cut my friends hair and my own.