At the beginning of the game, you'll reach a beach party with your friends. One of them will suggest playing "Truth or Slap"
Clarissa will ask something to Ren, and you will know that he is lying, because he just told you about it. So, in your next dialog option pick "He's lying"
and then "He told me..."
. After that, you'll have the chance to slap him. Get near Ren and push
. Follow this video if you have trouble:
Achievement unlocked! And you just had to slap your best friend in the face!