There are a couple of substitutions but probably your best bet is simply this:
For every cup of cake flour use 1 cup of all purpose flour minus two tablespoons. Basically one cup of cake flour is equal to 7/8 of a cup of all purpose flour (hence the minus 2 tablespoons).
Some substitutions will say to replace those 2 tablespoons with 2 tablespoons of corn starch. However given that cake recipes usually contain 2 cups of flour or more, the addition of that much corn starch could adversely affect the texture of the cake.
I've only ever had to find the substitute for a/p flour when I've only had cake flour.
But according to this site http://www.joyofbaking.com/IngredientSubstitution.html you can use 3/4 of a/p flour plus 2 tbsp of corn starch for 1 cup of cake flour.
However that seemed off to me, so I found this site http://frugalliving.about.com/od/makeyourowningredients/r/Cake_Flour_Sub.htm which says basically 2 tbsp of cornstarch in the bottom of a 1 cup measure and then a/p flour to make a cup. Basically 7/8 cup of flour + 2 tbsp.
Still not sure I went to my ancient Joy of Cooking (my newer one doesn't do Substitutions and Equivalents) and she just has 1 cup less 2 tbsp of a/p flour = 1 cup of cake flour.
On line I found that adding the corn starch will improve the texture. Also, if your recipe calls for sifted flour, be sure to sift the a/p flour before measuring.
You will get a difference in your cake depending the flour. But I have found that Gold Medal AP bleached flour works just as well as cake flour.
And yes to adding less AP flour like another poster said. And yes to the cornstarch thing also, but sift and stir a lot. Sift about 6 times and stir for about 30 seconds.
But I made cupcakes with cake flour and the GM AP bleached flour and the unbleached flour. The cake flour and unbleached flour cupcakes were identical. You couldn't tell the difference. Not so much for the unbleached/cornstarch cupcakes. I will not be using that kind of flour for cakes again.
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