For this achievement a specific gender is not required. If you are playing as a male character then this could either involve Ashley or Liara. If you are female then this can involve Kaiden or Liara. If you plan on getting this achievement with Liara then you must complete the mission: Liara's Dig Site, before you complete the main storyline (Feros, Noveria, Virmire). You will know when you complete the mission because you will gain the achievement: Search and Rescue. If you don't plan on getting it with Liara then that mission is not required. You must start by either completing Feros or Noveria. After the first is completed talk to your chosen character on the Normandy and follow the conversation path that sounds the most romantic. After the conversation is over complete the next planet and repeat by following the conversation that is the most romantic. After the second planet is complete, the planet Virmire will unlock. Complete Virmire and do the same again. After this you will unlock the final level of the game, the planet Ilos. On your way to Ilos your chosen character will come to your room and your achievement will unlock and then enjoy the cutscene that follows =).