You basically cut some small plywood spacers so when your arm rail is sitting in the miter saw (compound), it is oriented the same as when it is in it’s final installed position.
Below are two photos, one showing how to do it the WRONG way, the other showing the RIGHT way using spacer supports.
Note: I suggest using something more narrow that the 1×4’s I found lying around. 1×2 spacers will allow the work to be closer to the fence, which is safer and provides better accuracy.