You go to the carrier you want to use, bring a bill or something else with the account number of the account you're currently on, and when you sign up for their service, tell them that you want them to port your number from your current carrier. You don't do it, your new carrier does it. You have to keep the old account active until they do. (Porting the number out of the old account cancels the old account, so you do absolutely nothing at all, except tell your new carrier that you'd like them to port your existing number.)