Cutting out shapes inside cement board
There is no real experience needed when cutting an opening in cement board. I've seen openings for faucet handles created with nothing more than a well placed hammer strike (one hit each hole!). What ever tool you have available will suffice. I've drilled perforations with a masonry bit for a single shower valve. Oscillating tools with a carbide blade work. The primitive method is to cut the mesh with a utility knife and then force the board to break along the cut line. For a triangle cut not along an edge, mark the outline with a pencil, and drill a 3/8 inch hole at each of the 3 points. With an aggressive saw blade follow the lines to each 3/8 inch hole. If the edges are to jagged smooth with a rasp or  a whet stone.