They are a bit more user friendly about this on aa.com. So you will be able to choose your seat a bit before payment. So best to make a booking without logging into Aadvantage, enter your BA number of the first substantive screen, and it should give you access to the seat map. If that doesn't happen, or if you rather do it later, there seems to be a feature that you then have to wait until ticketing - typically a few days later - before going into their MMB system and amending.

Don't amend in the 24 hours before departure if you can. Don't forget to order your food too, while you are at it. And make a written note of the locator, their email confirmation system is sometimes very slow.

Also note you can make a booking and hold it for 24 hours before paying (actually it can be 36 hours depending when you are doing this). This is a good idea since it allows you to double check you haven't made an error with dates. Your seats will be held too.