Usually an established Chapter that shares the same geneseed is chosen to provide the newly-raised Chapter with its command structure; this includes the CM, CL, Rec and also a number of Veterans to act as trainers and Company Captains as the new Chapter's numbers grow.
Eventually, the new Chapter is big enough to start operating alongside other forces, who will take the lead in combat ops. From what I infer, it will be during this phase that the Astartes who will become the first CM and other leaders of the new Chapter will show themselves through bravery and tactical/strategic brilliance. At some point, it will be determined that the new ones will have reached a stable size and can govern and perform combat and campaigns themselves, at which point the old ones go back to their parent Chapter. I believe that this comes from one of the IA books, possibly number 1.
Last edited by Commissar Ploss; 11-02-10 at 07:35 AM.