A split image rangefinder focusing screen is a quick and simple accessory to install.
5D Mk II focusing screen
Turn your camera upside down.
With your fingernail, pull latch (1) towards the front of the camera to release the catch (2) on the focusing screen frame holder.
Hold the focusing screen frame holder up with your finger and remove the existing screen by grabbing tab (3) with a pair of tweezers and lifting the screen out.
Hold the new screen by its tab with a pair of tweezers (4). The spilt image side faces the prism.
Hold the frame holder up with your finger and set the new screen in place with your tweezers.
Lower the frame holder and gently push down on the frameholder so the catch (2) snaps into the latch.