Good news, there's a diagram for this on page 21 of the FireStudio Project manual: https://pae-web.presonusmusic.com/downloads/products/pdf/FireStudioProject_OwnersManual_EN.pdf
The diagram's a bit blurry, so I'll explain what's happening here:
You've got 8 general purpose outputs (or 4 pairs) on the back of your FireStudio Project. You can feed those stereo pairs (1&2, 3&4, etc.) out of your FS Project into the "Stereo Ext. In" inputs on the back of the HP60. So outputs 1&2 from the FS Project go into Stereo Ext. In 1 on your HP60, Outputs 3&4 go into Stereo Ext. In 2, etc.
If you need more than 4 mixes, you can just feed one general purpose output from the FS Project into the HP60 and press the "Mono" button on the front of the HP60. In this way, you'd be able to use all 6 Stereo Ext. In inputs on the back of the HP60 and still have 2 general purpose outputs leftover from the FS Project (which you could actually feed into HP2s or an HP4.)