Step one: Grab a measuring cup. Scoop your all-purpose flour. Level it off with a knife. No cheating!
Step two: Scoop out exactly two tablespoons of that flour. Toss it back in the bag. It’s fired.
Step three: Add two tablespoons of cornstarch to the remaining flour. Now you have a flour-cornstarch hybrid.
Step four: Sift this mixture into a bowl. Do it at least three times. Toss it in the air like confetti. Aerate that powder!
That’s it. You just created a cup of homemade cake flour. High-five yourself.