Stuart, I have the PPS spray gun, but have not used it enough to make friends with it. I am a little unclear on exactly which part you are asking about. The lid on the cup? Press the top on firmly and screw the ring on. The assembled cup onto the stem on the gun? Turn the gun upside down with the cup sitting on the counter top and Press the stem down firmly and turn it a quarter turn.
Go to You tube and look it up, lots of good videos there.