How The Caliphs Were Chosen

Today everyone is talking about the caliphate. Recently a supposed caliphate was established. But can someone just declare himself Caliph of the Muslims? A true khalifa is chosen by the learned elders of the faithful (which presumes the existence of a single unified body of Muslims which we do not have today). The study of the "Rightly Guided" caliphs and their policies is part of our usul as well. "Follow my Sunna and that of the rightly guided caliphs after me. Bite onto it with you molars." Their policies are evidence of the highest order for us because they were backed by the ijma of the Companions.