The round metal ring goes over the neck of the tank.
The clunk line goes on the end of the black plug assembly that has the o-ring.
Lubricate the o-ring (vaseline, cooking oil, castor, etc) and the inside of the neck.
Slide the plug/clunk assembly into the tank. The o-ring makes for a tight fit. It takes some force and sometimes a little twisting motion to get it to pop in.
The molded plastic nipple at the top of the tank is for venting or muffler pressure.
The other nipple is the fill line for a 3 line system. It is not drilled all the way through because many people do not use 3 lines. If you need to use it you should drill it open and rinse out the tank before installing the clunk.