First step: what do you want to make it out of (what material?) I would definitely take a look at some of the threads in this section because multiple people have made their own armor. Check out how they did it.
Lizard there is a string on this forum that sells an inexpensive set of already made chest armor and knees made from sintra for about $40.00. Good starting point, plus you could get a lot of expertise practicing painting, layering ,shaping, dent placement, dremel skills and everything else about costuming by saving and purchasing same. You might consider developing the detailing skills, prior to the manufacturing skills first.
depends on what you make it out of. if plastic, use a lighter, heat on the back side of the plastic. then with a sharpie or some other oblject with the right size and shape push the plastic in. Just don't touch the plastic with the lighter. you want to soften it, not melt it.
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