This is the flour mix I've been using lately. It's cheaper than a pre-made mix if you buy the ingredients in bulk. Substitute it cup-for-cup for regular flour in most cake and muffin recipes. No need to add additional guar gum or xanthan gum to the recipe you make, as it's included in the correct proportions.
Gluten-Free Cake Flour Mix
3 cups brown rice flour (fine ground)
1 cup potato starch (not potato flour)
1/2 cup tapioca flour
1 1/4 teaspoons guar gum
Sift ingredients together, combining thoroughly. Use in place of flour in cakes and other baked goods. Store in airtight container or ziploc bag in refrigerator.