Personally, I look for other people who have built armour before and pick their brains for info. If you are new to building armour, your best bet is to do just that. If there is noone around to do this with, best read a lot of books, and look at lots of pictures of armour. Here are some links to help you get an idea of where to begin.
ARMOUR BUILDING LINKS
I have been making patterns for some of the armour that I have made. All of these come in 2 flavors, either in jpeg format (*.jpg), where they are not in scale, and DeltaCad format (*.dc) where you can manipulate them if you download DeltaCad.
These patterns have been used with plastic barrels, with varying degrees of success. The knees and elbows tend to lose their shape over time, so they should be reinforced with metal strips to force them to hold the U-shape they should have.
Body (like coat of plates)
Shoulder
Inner Elbow Lame
Outer Elbow Lame
Elbow Cop
Inner Knee Lame
Outer Knee Lame
Knee Cop
Pembrook Helm
Spangenhelm