How do I move songs from iTunes to an SD card

Actually, I have sucessfully done this in the past, but can't remember what I did. I want to copy songs from iTunes onto an SD card so that I can play them on my Garmin gps. If someone can walk me thru this I'd greatly appreciate it. Do I "select-drag-drop"? or do I "select-export"? Can I do this with an entire playlist? The files need to be mp3.

Like I said, I was able to do this the first time I tried just a few weeks ago, but I can't remember how I did it--I'm feeling really stupid now. I have an album's worth of songs on the SD card and I'd like to add a bunch more before I go on a road trip.

thanks!