You can go in either direction with the armor, there are some great armor makers here on the boards and you can get a set at a really reasonable price.
If you want to do the home made, as others have pointed out, heat it up until it just starts to bend and then form it. Shoulders don't need much of a bend, so just making them even is a good thing to shoot for. Knees are pretty similar in that the forming is more of a "u" shape when you look at them from the top down as it would fit on your knees. My chest plates, I literally heated one side at a time and using heat resistant gloves so I didn't burn my hands, pushed the plates to my chest to help get the shape and the bends in the right places. Highly suggest putting on your vest first as using that as a layer of protection from the hot plastic. If you are going to do the formed breast plates, you'll have to watch the forming over complex curves as the plastic can form folds, and if you get the pieces too hot, they will warp and create sharp bends/divots that you have to reheat and gently coax back into shape.
Another way is to use a bowl with a rounded bottom on the inside and get the plastic heated up and then push it gently down into the bowl, usually using something round to push the plastic with, to form your cups, remembering to leave enough material for the bottom corners of your plates and when you are forming plastic into cups you have to remember that the flat shape will be bigger than you think.
A tool that a lot of costumers use is to make a duct tape dummy of themselves so that they can form the armor to their body shape and is usually a lot easier than trying to push the items onto yourself.
Anyway, looking forward to seeing your armor progress.
