thanks guys. first: of course plan out your armor.  try Vanity to gather your thoughts on the pieces you want to do.  
second: which piece to start on.  the boy armor is great to start off with, but I would suggest a helmet since it is more iconic, and can be worn by itself.  After i start a helmet base, then I'll begin to put the time in for the body armor. I generally start off with the torso since i can scale the rest of pieces to it, and its the largest part, but if youre just starting out with foam, I suggest starting off with a bicep piece or forearm. those are small and contain every bit to practicing your technique before you use up too much material.  
third: use an X-acto knife, and have plenty of blades because you will be flying through them. 
fourth: thin layers of hot glue, and lots of pressure on connecting glued pieces for good seams. 
fifth: foam has its own thickness, use that for detail
and last of all: take your time, rushing will call for unsatisfied product.  
if you have anymore questions, just PM me
Danny