Measure by weight if you’re fancy. One cup of all-purpose flour weighs about 140 grams. Remove 15 grams, add 15 grams of cornstarch. Boom. Perfect ratio.
But if you’re a cup-measuring rebel, that’s fine. We love rebels. Just fluff your flour before scooping. Don’t pack it in.
And remember: baking is just edible chemistry. This is your first lab experiment. Wear an apron instead of goggles.