First make sure rifle is unloaded.
Then remove receiver from the lower.
Place the lever over the bolt catch.
Place backing plate behind the bolt catch.
Install and tighten screw with provided Torx wrench.
You can now engage and disengage the bolt catch with your trigger finger for rapid operation of the bolt catch. (Note that this lever only works on standard AR15s. Not designed to work on a .308 platform.)