There are many excellent GF cookbooks, you might want to check your local library or even a used bookstore. Having been on a GF diet for more than 10 years, I have learned the best substitute for wheat flour is a combination or other flours (rice, tapioca, cornstarch, potato starch). You will need to add xanthan gum if baking. The flour mix formulas can be expensive due to multiple ingredients, so many people prefer to buy prepackaged mixes. I like GF products by Bob's Red Mill company, but there are other good ones also. Best of luck. Your celiac relative will sincerely appreciate your effort!