Hello and welcome, not a lot of guys make armor from scratch it's a lot of work, most purchase ABS plastic kits, lots of info in the Getting Started Section.
Those that do make their own in various ways, one is from pepakura files, then you have to coat in resin and bondo, lots can be found on the RPF, here's a helmet build http://www.therpf.com/showthread.php?t=122443
Some even sculpt their own plugs and vacform themselves, it's also a very time consuming process and can be costly