Transfer is simple as other posters have shared. Just follow the purple signs that say "Flight Connections" and take the bus to T5.
Don't waste time getting to T5. We arrived from ORD at 6:40A to connect to our 8:50A flight to EDI and only had 15 minutes to spare prior to boarding commencing. Upon entering T5 you must get your picture taken which can take a long time depending on demand. We spent almost an hour waiting to complete this.
Another tip is to print your boarding pass from ba.com (or obtain from AA, if possible), otherwise you'll have to queue in a separate line prior to security.