Also requiring some preparation, this achievement can be a pain to do. Throughout the story, girls will approach you, asking if you want to go out sometime. You can only get a girlfriend after one of four points in the story, and there are 6 possible girlfriends you can get, all depending on how much style you have. The girlfriends, style requirements, and how far you must progress into the story for them to approach you is as follows.
1. After you help Luda in the beginning.
2. After the cutscene that introduces Troy.
3. After you get the penthouse.
4. After hitting the town with Luda.
Nina: 1 style
Liris: 20,000 style
Mayra: 35,000 style
Summer: 50,000 style
Christin: 75,000 style
Melyssa: 100,000 style
Many people have trouble getting the girls to approach you, as the game is somewhat strict when it comes to style. You must be somewhat over the style requirement in order for a girl to even approach you, so you will need to buy some jewelery. Not a lot, but just enough to get you over the requirements for one.
After those certain points in the story, girls will only approach you after fights at either the Club or the After Hours. Don't worry if they don't talk to you right after those points in the story, as more often than not, they'll approach you after your next club fight.
Pleasing the girls themselves is the easy part. Right away, always give them what they want, and never say no. Also, when you finally move from the dorm to the Penthouse, always check your messages. From then on, your girls will occasionally leave a message saying they want to come over. This will show up on your appointments, and is very important if you want to keep them happy, so always pay attention. If you've kept them all happy, by the time your fourth girlfriend finally asks you for something, you should get the achievement.
Due to how you have to spend for this achievement, it's nearly impossible to get both this and KaChing Bling. So before you start your playthrough of Build a Label, plan on which one you want to go for.
Huge thanks to Demon Ninja for listing the story points when girls approach you, they're style requirements, and all girlfriends.