The walkthrough is basically: Pick ONE Love Interest. Save before each choice. Choose then click ahead for a bit (the points aren't always instant) and see if or where the points go up. Reload and check all options before saving. ONLY pick the choice that increases your main LI. Restart the game for each of the other 11 LIs.
That's right. To see everything, you'll need to play 12 times. Judicious save games and use of "skip read text" will save on time, but there it is. As posts above have said, that's how the dev wants it, so that's that.
It's not terrible though. So far, I have 3 saves. Tiffany. Mrs Smith. Joan. In each of the cases, all of the other LIs have MAX points. They'll need a new save path when they branch off (i.e. when 2 LIs are competing for points during a choice).
Now whether you like that in a game, or not, is up to you.