First, you have to trigger Happy. In the beginning part when Liz does not want to go to work, tell her to go to work. The path will now become Happy.
Continue the dialogue until you reach the scene where Liz will ask you if she will go shopping with Sam or shopping with MC. Tell her to shop with Sam. Continue the conversation until you reach the part about guys ogling Sam and Liz. Select the choice "That is kinda hot!". You are now in the Happy NTS path.