How can I get started on a winglet design for a small remote controlled aircraft?
For best results, put good flaps on the wing and forget using a winglet.

If you insist on bending the wing up at its end, just continue with the tip airfoil and select the incidence such that your circulation distribution over the wing and winglet will become elliptical when you plot it in a straight line. The small size of your airplane implies that you will not need to unload the tips to reduce root bending moment.