Hi scarecrow, crow62 here, on mandos the appropiate term is flight suit, the undersuit is used on clone troopers and some fem costumes;
Asumming you are planing to make a mandalorian like Jango, Bobba, etc. the templates are the printed paper pieces to draw the armor pieces on metal, plastic or other material choosed for you. The finished armor parts are attached to the vest and flight suit with velcro, elastic, etc.
About helmet you can do by your self using pepakura, aluminium or if you have the skills with fiberglass.
The mos used way is to buy your helmet on the sales section, there are many makers and sizes.
In the forum you will find many armors WIPs (Work In Progress), you will find many answers there, of course any other questions will be welcome.
Giovanni