1. Select "Edit" in the Monitors section of the Master control area (bottom right side of the master control section)
2. In the Monitor Edit screen, select "Solo"
3. Select "Edit" in the Solo section (just below the Monitors section)
4. There are a few settings, but the most common are: Set "Latch", and "Pre Fade Listen".
Now, you can use the Solo button on one or more channels at the same time. All soloing will be PFL. Adjust monitor head-phones as needed. Also, you may need to adjust the "Solo" volume level (the Solo section) up a few db.
I did a video on this some time ago:
Hope this helps.