First, and most importantly, you need a game plan. Think about what you want, then sketch it out. It doesn't have to be perfect, but you need something to reference while you work.
Using this as a guide, I took my measurements and set up my mannequin to be as close as possible. Err on the side of too big. If you don't have a mannequin, make a duct tape dummy. I'll make a tutorial on that at some point if needed. Personally, I find it easier to make my templates using fabrics, as they mold better. I wait until the local craft store has a 50% off one fabric item coupon, then buy 10 yards of the cheapest fabric they have (generally muslin or broadcloth). Drape fabric on mannequin/DTD.
Then, using chalk or pen, sketch your design.
Repeat until you have all the pieces you need. Then, trace onto foam and cut out.
Test fit them to make sure they will work. Then, you want to do a glue wash (50/50 elmers glue and water) on all of the pieces. This seals the foam. After, you can choose to cover in resin, fiberglass, epoxy, or plastidip in order to make the pieces sturdier and more rigid. Do all of this BEFORE you put the pieces together. You also want to paint while they are still separate. I will add a part two once I finish and have more pics.
Until next time!