Enjoy the Concorde Room however here is the info you need.

You need to disembark from your domestic flight and assuming that you got AA to through check your checked baggage, you then need to leave the terminal and take the Airtrain in the Clockwise direction from Terminal 8 to Terminal 7.

You then should go to the First Class check-in and let them know you've arrived and get a boarding pass if you don't already have one, and then go through the First Class/Emerald OW security to get to the departure area, where the Concorde Room will be on your left.

There are not easy ways to connect at JFK other than doing this, unless you are connecting in the same terminal, which you are not in this case assuming your BA flight number is 178, 182, 112, 114, 116, 174, 176. If your flight number is BA and four digits, you are on an AA flight which is a codeshare, and you will stay in the AA terminal (T8) and not be able to use the Concorde Room.