Chinatown Detective Agency

Once you have completed the first 3 missions as described in the other guide you will return to your office.

You will be told about paying rent, saving your game etc and be told you can now choose to work exclusively for one of the 3 clients.

I chose Rupert to unlock the achievement and then immediately pressed start and reloaded the checkpoint to do the same for the other 2 people (if you want to unlock all 3 of these achievements right away)