OK, here's the thing with this game that is different from other games for all people asking for a walkthrough.
If you miss scenes, or mess up with a girl (obviously depending on how much you mess up or how many times). Most of the time, there is a way to make up with the girl in question and get her back on your good sides. With some of them it's easy to get back on their good side (i.e. Yvette), with some the path to their hearts or into their pants is a long one (i.e. Kirsten)
In both cases, you have multiple chances to advance with the girl, and even if you make a mistake, you can make up for it at a different occasion.
Let's take Yvette as an example. She can become your GF quite easily. But then she becomes a bit demanding and Jealous. So you can ignore her, she get's mad at you, but then next week you spend some time with her again and she forgives you are all good again. You can go out with another girl, she hears about it and confront you with it, you buy her a present spend some time with her again and all is good. OR you keep on ignoring her or tell her to fuck off and she break it up with you. Its a bit of common sense.
Some girls you can not get quicly. They might prefer someone else (Melissa) or be in a relationship (Tatiana). With these girls just like in real life, you need to butter up over a long period of time to try and get them on your side. You also need to be aware of clues you get (For instance, Tatiana's boyfriend, Connor has a crush on Kirsten, your Ex-GF...And Kirsten is into swinging depending on her partner's approval....So there is an obvious opportunity in there somewhere)
You also can not easily get all the girls... A lot of times you need to chose who you want to spend time with.
Some girls hate each other, so you can possible get into shit there (Michelle and Melissa)
Having said all that I managed to sleep with the following girls in one playthrough so far.
Madeline
Harley
Meadow
Martina
Yvette
Luana
YanMei
Catherine
Roberta
Pinky
Ashley
Michelle
Zoe
Hannah
Belen
Some of these were a one night stand, some were as fuck buddies, some as a porn actor/director and some a proper boyfriend.(I believe if I had "Coke" on me at the right time, I could have had Melissa as well. Everything for a fuck right?)
So TLDR; Don't worry too much about a walkthrough. You can not mess up too much unless you continuously do it to the girl you like. Some girls are just not that into you (yet) is probably the reason why you are not making headway yet.
Also common sense.
For instance: