I’d hope your monitors are being fed from an output rather than an input!
OK, so headphones are plugged into the headphone socket near top right of the front of the desk. Just below this is their volume control.
On each channel (and the master) the lowest button, marked PAFL will switch on that channel to the headphones. Simple as that (almost). Pressing it again turns it off in the headphones.
I think the default settings of the desk allow only one thing at a time, so if you are listening to channel one, then press the PAFL button for channel five, one turns off, and five turns on. This behaviour can be changed in the desk settings and is called “Additive Mode”. I suggest you look at page 61 of the manual for all of the options available.
LR to PAFL is a function that puts the main output into the headphone when nothing else is selected – IIRC this is turned on by default.