I don't know too much of photoshop, but here I go:
First, you take a picture of your forearm, leg, chest or the part of your body you wish to make.
Then you open pepakura and in the left window, rotate the model with the right mouse button until it looks the same angle as your picture. Then take a screen capture of this window.
After this, open your body picture in photoshop, gimp or your favorite program and create a new file and add two layers. Put your body picture in the bottom layer and the pepakura screenshot in the upper layer.
Finally, using a magic wand tool (or something similar) cut out the excess of the pepakura screenshot until it looks like this:
This picture was one of my first experiments with this .... ahem, "technique". In this case, the chest piece was very very small.
This method is (obviously) far from perfect to get a exact scale, but using your own body measures as reference, it is useful to give an overall idea of how the piece or armor will be (too little, very big, etc) once you build it.