A relatively simple Trophy to earn. Simply go to any area and press the Circle button over any of the flags marked on the map for that zone. Once that's done, a heart will show up on the map, denoting the location of a potential mate. Go up to that spot and bump into the female of whatever species you're playing as. If she's just not that into you, simply go out and rank up until you hit "Boss" status.
Once you've got the female's approval, take her back to your "nest", which is also marked on the map. You need to physically step into the nest and press the Circle button to mate, thereby earning you the Trophy.