[Stiglr]

Basically, the best way is to find a wing filet (cross section) and extrude it outward. Your choice as to whether to "cut the wing filet out of the fuselage" or to have it intrude into the body of the model.

While extruding, you'll want to plan ahead to make the separation of flaps, ailerons and gear doors easier.

There are a few ways to get better wingtips, but they're a bit too involved to go into in a short post.