To do this you need to create and save 5 synchronisation files from your own music. To do this you go into Single Player mode then into the Beat-O-Matic and select Analyzer. Once here you need to play the song you want to synchronise. The song file can either be saved on your 360 HDD, on a computer you've linked to the 360 or a mp3 player or USB drive plugged into the USB connection on the front of the 360.
Hit the Guide button and go to the music player and find the file you want and start playing it. Quit the guide and now you have to listen to the whole file whilst the game analyses the BPM. Once this is done it'll ask you if the BPM it's picked is correct, just keep telling it Okay (or tweak it if you want) and finally it'll ask to save. Repeat this until you have 5 custom files :D