I won't give detailed instructions for vacuum forming here---that's for another instructable I'm working on.For more information on vacuum formers and...
Step 10: Screw the nipple to the platen flange
Wrap several layers of PTFE tape (teflon tape) around the threads of the pipe nipple, where it will screw into the floor flange. You need several layers, because the tape is very thin, and you want to build up enough thickness to fill any mis-fits in the threads and make a good seal.
Now screw the pipe nipple into the flange, hand tight.