The path of sound ALWAYS breaks down into two parts. Gozinta and gozouta. Combine this with "wire is just wire" and you now have the basis for understanding all signal flow period including digital.

If you have a mixer and you want to run that into a headphone distribution amp you have to figure out how to gozouta the mixer and gozinta the headphone amp. Often there is more than one path.

Now take a look at your mixer and figure out the gozouta part. The other will be easy.

Your ART unit has a single Stereo In.
So you will probably need some sort of L/R TS to TRS cable or adapter.

Something like this:
http://www.sweetwater.com/store/detail/STP201/

No, you want to mix with speakers and then listened to it through headphones. Not the other way around. That's a mistake. A beginner's mistake. If it sounds good through speakers, it'll sound good through headphones. It sounds good on headphones, it will sound like crap through speakers. Headphones do not present the proper mixing environment necessary to accomplish a quality mix. It's for overdubs & private listening.

You can mix through headphones when you learn how your headphones relate to your speakers. Not the other way around.

I can do both after many years of experience
Mx. Remy Ann David