When putting tile in your bath tub or shower. Its important to make sure you have the right preperation. The drywall and your backerboard need to be the same thickness. In addition the walls must me flat. Shim the walls to ensure your backerboard is completely flat. Tape the seams with fiberglass tape and thin set. Use good backerboard screws to attach the board to the walls and recess the screws and cover with thin set. Leave a gap between tub and board, caulk with silicone caulk. Now your ready to tile.