In the old days it was pretty easy you just had to do the written and practical test, but since 2014 they've changed the rules. Now in order to take the ATP multi written test you have to do an ATP-CTP which is kind of like an MCC, and the really annoying thing is there is no exemption even if you're already type rated. I think they cost $5-6000. After that you can study for a week and take the ATP written. You can take the check ride in anything from a duchess to a 777.