there should only be 4 parts to the typ. 4st muffler.
the actual muffler.
the pipe that connects the muffler to the engine.
and 2 locking nuts that screw on to each end of the pipe.
thats it, the YS doesn't have a pressure fitting because its pumped,the manual doesn't cover it not sure why?

so without seing a picture of what you are talking about,here is what you should do.
this take the 4 muffler parts and lay them out in front of you first pick up the small connector pipe and screw the locking nuts on each end as far as they will go,then screw this assembly into your engine.
then take the muffler and screw this into the pipe also as far as it goes,holding the muffler with your hand tighten the lock nut at the engine side tightly in the position that works for your plane.
then use a wrench to hold the muffler, and with a second wrench tighten the lock nut on that side.
it should be as simple as that. its taken me longer to type this than for you to do it.
well put Ken.
if any of the threads are stripped then the parts need to be replaced the thread pattern is very fine and care is needed when screwing the parts together