The easiest way to get this achievement is to date Mallorie's friend Michelle once she introduced during the story. Take her out on a date and when you drop her off you will have the option to ask if you can come up for a night cap. She will say yes after a couple of dates. Wearing the right clothes will also help you close the deal. Alternatively you can also find some dates using the online dating site built into the game. Get invited up and the achievement will be yours.