Perhaps there is a way to actually perform a true "downgrade" from a technical standpoint, but if you're looking to simply move from Yosemite to Maverick and you already have Maverick on an external HDD, here's what I would do:
*This is a brief guide to copying everything new since the backup that you'd like to keep, excluding Yosemite obviously, then turning your macbook's hard drive into a clone of the external hard drive once the backup has been MANUALLY updated*
1. Turn off your mac.
2. Plug in the external HDD
3. Turn on your mac while holding the "option" key (this will allow you to select the drive your computer boots from)
4. boot your computer from your external HDD (it'll probably be the orange icon incase you aren't...