For you to be a captain you must be in a squad and at least 2 captains must recommend you to be a captain when a spot is open. Once you have been recommended you will be tested in 3 categories.
1.Captain proficiency test (隊首, taishu): A test which requires the ability to perform Bankai. Nearly all Shinigami become captains using this method. At least three existing captains, including the Captain-Commander, have to witness the test.
2.Personal Recommendation: To have personal recommendations from at least two captains and approval from at least three of the remaining eleven.
3.Trial by Combat: You must defend yourself against a captain (ANY Captain, your choice. No captain is to say for that person to pick them. If so they will not be captain or you be punished.) The Captains Of the Gotei is to examine this event and give there rate and opinion if needed.
This is my final say so