The Supreme Court has ruled that burning an American Flag is protected by the First Amendment.
Donald Trump wants to criminalize flag burning butt any such law would be patently unconstitutional.
So, here are a couple of laws (that do NOT conflict with the First Amendment) that might help resolve the issue.
All American Flags sold in the United States must be made of fire resistant material.
Anyone convicted of assaulting an individual who is burning an American Flag must pay a ten dollar fine.
UPDATE: Comrade Vlad's dog suggested adding a clause to Law1:
All American Flags sold in the United States must be made of fire resistant material and must be made in the United States.