My understanding is that once you configure an external drive to be your backup drive, once you reconnect it to your machine, TM should automatically start a backup for you.
Haven't tried this since I have a Mac Pro, but that's what it's supposed to do I believe.
I'd let it do the initial large backup first. After that, the backup process will watch and keep track of all changes, and wait until you plug the drive in again to copy them over.
-Kevin