Assuming you don't have any cake mix, then you basically need to concoct the equivalent in flour, sugar, leavening and salt, plus the extra flavors like cocoa powder.
this site looks about right, http://www.budget101.com/cake-mix-recipes/make-your-own-chocolate-cake-mix-565.html
Notice, however, that they're adding powdered milk and then hydrating with water. Depending on your particular recipe, that might not be what you want to do. It would be trivial to leave it out.