Best way to add a new instruction

Hello,
I am pretty new to modifying LLVM, so to get a good understanding I wanted to try duplicating an already existing instruction (like branch) and give it a new opcode (so by the end llc will accept an instruction with fbr opcode and produce the same result as if it was a br instruction). Can I get some guidance on what will be the best way to add this functionality?

Thank you!