The function needs to be active in the radio, that is done with programming software, and then you have to do one of the following:
Short pins 8 and 10 on the mic connector.
or
Remove R491(labeled "self") on the TX-RX board.
Turn the radio off, then hold PF1 and PF2 buttons on the side and turn the radio on. If you have an old speaker mic, you can make an "FPP Key" by shorting pin 8 and 10 and attaching that to the radio (a-la JT-1000)
The process is pretty simple, just stepping through the options and setting them. It's all covered in the service manual.
I had a TK-290 at work that I used as my "shop" radio, having the FPP was handy for making changes on the fly. Doesn't replace programming software, but it's good for quick changes.