I would recommend picking an existing airplane you like. Change the fuselage shape and rudder fin to start. Keep the wing airfoil and shape same as the existing and also the moment arms the same.
You will probably be able to purchase foam wing cores and stab cores easily rather than cut your own. You may find you purchase these already sheeted.

Then build your airplane, from this you will see and learn what works and what does not work.
Designing a good pattern ship or any airplane is not an easy task, it is a long process to become good.

My $.02 worth.

Orlando