Before installing the new rear derailleur, apply some waterproof grease to the mounting bolt. Then thread it part way into the derailleur hanger, and rotate it clockwise to make sure the b-screw or tab is sitting behind the flat on the back of the hanger. Then fully tighten the mounting ...
The next step in the process of bike chain installation is to fit in the new chain starting from the rear derailleur. If your bike has a clutch, release the mechanism. From the back, keep tracing the chain through the derailleur. Do the same until the end of the chain successfully passes...