Arms are very different sizes obviously, so this is an instance where you are going to have to play with the templates to find YOUR size.
I tried to make the templates easy to understand and adjust.
Print out the triceps and bicep templates. You're going to need to print out 5-6 plates for the triceps and 5-6 for bicep. You'll know how many you'll use depending on your arm length. I went with 6 for the triceps and 5 for the bicep.
The first thing I did was cut out the 5 tricep and 5 bicep paper templates. then I assembled them. I held them to my arm and taped them together so I could see exactly what size I needed to cut the plastic.
I marked the templates accordingly and cut out the plastic. The bicep part wraps around your bicep and the tricep part around your tricep.
When you have a nice paper upper arm built and you're happy, take it apart and cut your plastic. I would cut one bicep and one tricep strip at a time assemble them, then the next. That way you can better gauge how much the thickness of the plastic comes into play when cutting the length of the next strip. Since they over lap, each one has to be a little bit longer to accommodate for the increase in size.
I cut mine out and test fitted them but didn't rivet them. before assembling I heated them up with a heat gun and gave them a little bend just to help me assemble everything.
Now just rivet each section together.
Test everything as you go to make sure it is all fitting together.
Once they are all assembled you can rivet each section to it's neighbor to form one upper arm section.
As you can see I made my bicep strips a little shorter toward the elbow to create a taper.