Many people start their first costume with thicker paper, like cardstock. My printer is bottom-fed, which means that it cannot take very thick paper. Also, since I have the tools and experience for cutting cardboard, I thought it made sense to do that.
Assuming that you've followed the instructions on the Pepakura tutorial on the 405th website, you should have many sheets paper with numbered components. Be sure to visit the sizing tutorial, to make sure that your armor fits. The sizing takes into account height, not weight, so if you're quite large, you may to do trial-and-error on the pieces that you think might not fit. Pepakura is recommended for beginners, so if you make a mistake, you can always go back and try again.
I've started off simple, with the right thigh. Each piece of armor can be downloaded from the 405th website, so you can build the armor one piece at a time. The thigh seemed like an easy start, and by and large it was.
If you've never built a model or been to architecture school, take note that you need to be as precise as possible. Only after years and a dozen models should you trust yourself to gauge what is "good enough". Being a little bit off, even by 1/32 of an inch, can compromise overall integrity, fit, and appearance. Despite my experience and attempts, even this first attempt at the thigh has resulted in a few seams not fitting together quite right. I had to compromise where one piece would fit with all the others.
Hopefully, with enough fiberglassing, the gaps will smooth out. I've never fiberglasses before, so I don't know how that will go. What follows are pictures of what of the thigh that I have completed. It's not done yet, and the missing pieces are obvious.