I also went in playing my first file with the same goal of assembling a harem. I guess I can say I've succeeded. I've maxed out all 4 girls relationships a few months ago. I guess I just need a big get together night and watch the drama unfold as the 4 NPCs talk to each other and I get swept away in whatever nonsense they come up with.
You don't need any tricks to get them to sleep with you, early on. I got my first character laid three times on the 1st in-game day between Celandine and Rosalyn and I wasn't even aiming for that. I just wanted to increase their relationship bar to get a good start with them, so a bit so I talked to them a lot and it kind of just happened. In retrospect it's kind of hard for it to not happen. Celandine will throw herself at you. Rosalyn just wants some company and dinner before she's game. Mirel is almost as promiscuous as Celandine, but drinking with her before hand is a good way to get her interested. Oriana is the only one of the 4 who was difficult for me to win over, but I also accidentally made her mad at me near the beginning which basically reset her relationship bar. Thankfully, I found out something she likes also on accident. That really helped win her over. She's fun to have around, especially with Celandine. I love their banter.