back to the regular programming
I'm thinking a routed groove that would allow the arm rest to slide over the side panel. How would you do that? I'm thinking a dado bit with a bearing that would follow the curvature. You'd have to hand chisel the intersection where the top meets the curve. Personally, I like to make the joint itself structural rather than relying on dowels, biscuits or other means. JMO.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)