I'm having a real hard time with the learning curve of this game.

I'm playing as either Klingons or Dominion (using this mod - http://dondon2k8.niceboard.com/plug-ins-f2/glorious-empire-productions-t34.htm).

I've been able to take over Denmark and Gorn, but ultimately my prestige falls and they become hostile.

Even as the Klingons, my home planets start as allied but within a few turns I end up being hated by them.

I cannot figure out what I'm doing wrong.

Usually you lose prestige by destroying ships aligned to that planet, or using illegal weapons. And make sure you drop your flag that you start with so you can actually own the planet. Or could just be something with the plugin, never downloaded any to use those so don't know, that could be part of it.