If this is your first playthrough, just progress through the story until Alex and Julia go diving and you gain control of Fliss.
If you have already beat the game and managed to miss this achievement (Like I did!), go to Scene Selection and load "Uninvited Guests".
In the beginning of this Chapter, Conrad tries to flirt with Fliss. Choose these dialogues:
Relaxed
Self Assured
Uninterested
After the short cut scene, go to Conrad and then press A to talk to him. Now all you need to do is to select these dialogues:
Stern
Curious
Romantic
Good Luck :)