Assuming you're using a master bus channel routed to MAIN, just add a
send from that master bus channel to analog out 1&2. Make sure you've enabled analog out 1&2 in the audio configuration so it shows up in the console's outputs section. You might want to change its 'friendly' name to "Phones' so you know what you've got hooked up there. It's fairly straightforward. I use MOTU interfaces and have no problems with the phones output, though.