Make cake flour, from all-purpose flour

Sift 5 or 6 times.

For every cup of flour:
Remove 2Tbl. of flour, and replace it with 2Tbl.of cornstarch. Sift 5 or six times.

If you recipe calls for a fraction of a cup:
3/4cup -  4-1/2tsp. cornstarch
2/3cup - 4tsp. cornstarch
1/2cup - 1Tbl. cornstarch
1/3cup - 2tsp.  cornstarch
1/4cup - 1-1/2tsp. cornstarch

So if a recipe calls for 2-1/3cups cake flour.
Measure out 2-1/3cups of all-purpose flour.
Remove 4Tbl. plus 2tsp. flour.
Add 4Tbl. plus 2tsp. of cornstarch.
Sift the flour, by pouring it into a strainer, over a bowl, tapping the side of the strainer.
Repeat the sifting, 4 or 5 more times. Re-measure before adding it to a recipe, the sifting will increase the volume a bit. Dump any remaining cake flour, back into the all-purpose.