My wife showed me a picture of a bench on pinterest that she wanted. It was pretty simple. Two sides that were attached by a seat and back. No idea how it was put together, probably pocket screws from the seat and back into the sides.

I was going to put two mortise and tennoned rails between the two sides to support the seat and give it a frame structure. Im struggling because I usually mark for the mortise on both sides and chop half-way from each side. As you can see from the picture, the sides are already curved. the edges are also curved from spokeshave so I wouldnt get good registration with the marking gauge.

Any advice? Am I even going about this correctly or would another construction method work better?

Thank you!