If you're joining the 501st, find out what they say about costume requirements. First, define which Episode's costume you're pursuing: ROTS, ANH, ESB, ROTJ. Then select the Costume Requirements List for that particular selection. Here's the CRL link: http://www.501st.com/databank/Costuming:SLD_CRL
Yup. Often some kind of circular hole and slotting system. Look at the MR Rots page on Star Wars helmets. The strapping on the face plate is pretty clear. On top of the face plate there's a raised circular section (at the very top) and that locks into a corresponding hole inside the top of the dome. Similar systems on the don post and efx offerings as well as a lot of fan offerings; my old ghost host any helmet had the same sort of thing. Strap mask to face then pull dome down onto top of head. It's really quite straightforward.