Hi, I have included the following drawing to help you out. The red lines mark the curve points. (generally)
Here's the break down.
1) Bend the side fins - most likely need a heat gun for that
2) Lay the back over a large pot of boiling water. Let the steam heat the plate and holding the fins push the plate down while moving backwards to get a gentle curve from the fins up to the top of the back. (hoe that makes sense)
3) Dip each "shoulder" in the water as far as you can and then hand mold the shoulder piece to a curve that will match your shoulder.
4) Give the piece a quick lateral fold to get some curve to hug your shoulders.
5) Bend the outside tips of the shopulders a few degrees to match the slope of the shoulders.
** When doing the collar armor, heat it up and bend it gently over your stomach bottom point down. Then heat the shoulder tips and give them a nice bend to meet up with the back plate "shoulders".
Hope this helps.
Edit: Ooops, I skipped the part where I strip naked, dance around the boiling pot while sprinkling chicken blood and singing to the sintra gods.