Soar Tech 10 has a detailed, 50 page, illustrated article on the subject. It is available from the publishers of Flying Models Magazine. Soaring Stuff
http://www.soaringstuff.com/videos.html
has an excelent video on the subject that shows the process in great detail. Soaring Stuff is a part time business that is not noted for prompt customer service so, be patient with them.

As with most design questions, a conflict of objectives is part of the game. Strength and stiffness conflict with light weight. Where the balance should fall to resolve this conflict is a matter of the purpose of the aircraft and the associated design priorities. Cost, performance and ease of use conflicts also need to be resolved.

Light balsa, Rhoacel and dense closed cell polystyrene foam are used for core material. The order of expence is Rohacel, highest, next balsa and polystyrene foam lowest. Rohacel is easiest to work with, then balsa and with styrene foam you may have to hot wire cut your own sheets. Balsa is the least resistant to denting and rhoacel the most resistant.

The skins can be Kevlar, carbon fiber, E glass or S glass. Fabric weight from 1.5 to 3 ounces per square yard cover the useful range. There is a very wide range of ease of use, cost, strength and stiffness. There is also a wide range of epoxy properties available.