Usually the oxidizer tank has the injector bell already attached. Choose the orifice that provides the proper thrust for the rocket you are going to use it in. Insert the orifice, with notches for screwdriver pointing towards the fuel grain, into the injector bell. Using the ¼" flathead screwdriver screw the orifice gently until seated (do not over tighten). Now apply the 100% silicone grease (do not use Krytox for it may react with the HyEFX fuel grains) to the small 2-010 o-ring and insert it into o-ring slot in the injector bell in front of the orifice. Now apply the 100% silicone grease to the large 2-210 o-ring and insert it to the outer o-ring slot on the injector bell. Now take the fuel grain and screw it into the injector bell until flush against the injector bell housing.