We cannot unfortunately help with specific installations, since there are so many different aircraft types. FLARM is normally installed by a Part-145 or Part-M organization, and they usually have the competency for avionics and antenna installations. Antenna placement and installation is also governed by the PowerFLARM Core installation manual and the EASA MCA Minor Change documents.
However, we can give some generic advice:
On most aircraft, one AV-75 FLARM antenna should be installed on top of the aircraft, and one AV-75 antenna below the aircraft. The top antenna should be placed as far forward as possible (taking the specified restrictions into account), to maximize forward range. The bottom antenna should be placed below the aircraft, on a location that ensures free view behind the aircraft.
If e.g. the top antenna cannot be placed with a good forward view, it's also possible to instead place the bottom antenna with free view forward, and then place the top antenna to the rear. The vertical stabilizer might then obstruct the view behind the aircraft, but the bottom antenna will normally compensate for this.