Alright, listen up. In order for an unseated to rank up, that person must challenge the lowest ranking seated shinigami and then if you get that spot you must challenge the one above you. No skipping. For Arrancars, in order for a Numeros or a Fraccion to rank up, one must challenge the lowest ranking Espada and must defeat him/her. When ranking up within the Shinigami, you cannot kill. With Arrancar however, you must kill the Espada which has lost. With Regular Quincy, you must challenge the lowest ranking Legendary Quincy and so on. Once again, you cannot kill.
When a regular Bounto wants to rank, he/she must challenge the lowest ranking Legendary Bounto and if the Legendary Bounto is defeated, the victor must absorb or feed on the loser.