Cut out the next piece of leather using this pattern:

Then, cut out the plates from these patterns.




Again, drill or punch rivet holes in the plates, either using your own rivet pattern or mine. Then place
the plates on the leather like this:

Mark the leather, punch holes, roll the plates, and rivet on. Before finishing, place straps between the leather and the plates like this:

Last, either put buckles on the straps, or make your own buckles.

Below are 2 pics of the buckle I use. They should be sufficient for you to make your own. I used 9 gauge stainless steel wire. The circular part of the buckle is an inch across.

Assuming you have all the pieces done (vambrace, rerebrace, elbow cop), there are several ways of finishing the piece. You could wear all 3 pieces separately (works, but bad idea... However, the vambrace is cool by itself). You could attach the rerebrace and vambrace to a 3-5 piece articulated elbow (easy to do, but if your going for articulated steel you might as well wear full steel arms). You can attach the leather edge of the rerebrace and vambrace directly to the elbow cop (what I did in the pic. It works, but is better if you leave a little extra leather on the ends that need to be connected). Or last, and perhaps best, you can attach leather straps to both the rerebrace and the vambrace, and then rivet the straps to the elbow cop (this works really well, and allows a lot of customization for size). Additionally, you can add straps or points to connect these arms either to your spaulders, or to your gambeson.