Scott,
Standard guitar-type (1/4") plug is all you need.
The left output should give you left and right sounds combined. The right should give you only the sounds that are panned toward the right. Both at once is the best way to do it--even if you are running mono.
Be aware that some cords have stereo jacks. You will see a little ring around the plug.
Inserting this type of plug half way, or not completely could give you strange results.
Also, you can use the RCA outputs, but be aware that they are "wide open". You will need to control volume from a mixer.
I usually run my keyboard in stereo, using the 1/4" outs to go into a stereo channel on my mixer.
Then, I connect my digital recorder to the RCA outputs and get a good strong signal to the recorder.
DonM