How to design one's own aircraft
Hi, this is Ken.
I just have a few tips for you, because I am no that professional into designing. (not in specific orders and some of them may seem stray :/)
1. Use helpful 3D graphing tools such as "Blender" to create a sketch of what your desired plane would look like.
2. Find a suitable airfoil for your airplane, maybe at airfoiltools.com.
3. Build parts of different scales for the first plane, so it would be easy to make changes and adjustments, such as the vertical stabilizer is not doing its job.
4. Test fly a ton.
5. Try new stuffs. Maybe like me, using sanded, bent cardboard paper to cover up the edgy foam board fuselage (getting a smooth and round airframe from foam board is just too hard) and it worked out perfect both visually and aerodynamically.
Good luck.