I use tracing paper and a number 2 pencil to trace the parts with.

Then flip the tracing paper over onto the wood. Then, by marking over the reverse side of the tracing paper, the tracing of the part, which is now against the wood, is transfered to the wood. Then just follow the line with a saw.

I usually stack and pin ribs together, or any part I need two or more of, for cutting so that they come out uniform.

Maybe this can help someone else.