Cake flour is lower in protein than AP flour so it gives cakes a more tender crumb.
You can make a "mock" cake flour by removing 2T of your all purpose flour and adding 2T of cornstarch. I have not done this, though it is a generally accepted method.
But I'd probably just make the cake from all AP flour. I've done that a million times.
Less is not more. More is more and more is fabulous.