Open the patio door. Remove the two screws on the handle's faceplate using a Phillips head screwdriver. These two screws hold the patio door's outside handle together with the inside handle.
Discard the old handle's two pieces. Line the new handle's screw holes up with the patio door's screw holes. Mortised handles also have a piece on the back that fits through a hole leading to the latch assembly.
Insert the provided screws through the inside handle's screw holes. Check that the screws fit all the way through the patio door and into the outside handle's screw holes. Tighten the screws with a Phillips head screwdriver.