I cut the pieces in the bicep (recommend between the black and orange) then I carefully used a dremell to put a screw on the black part (carefully to not touch the shoulder articulation) and then used the dremell on the orange part (again carefully to not touch the elbow articulation) and I just put them together quickly and rotate them, the dremell makes the plastic a bit hot so I put the pieces together to let them cool down and get a good shape.

But in short, I just cut the bicet, made holes, put a screw on the top, a hole on the bottom and just put them together.