It's fairly simple. First put where the lens is under the notch, then slightly squeeze both sides and put the left or right notch under the groove and put the other end in the groove and release your hand and it will perfectly sit on it. It takes a little practice but, you will get the idea. Some folks force things in place and break things.
i have given up...i now use a rubber band to hold the Plastic cover..all fits in and i no longer fight with trying to make that gizmo work...