Best to start making costume armor?

A great deal of costume armor in the cosplay world is made out of foam, so I would start with Ted Haines's foam fabrication lessons on making a T-Rex.  While the subject matter is not the same, the tools, material, and techniques apply to all foam fabrication.  

https://www.stanwinstonschool.com/tutorials/make-a-dinosaur-t-rex-foam-fabrication-part-one

/Chris