Hey, the above is true and there is nothing to change on it. But there are ways, the most important thing is that you avoid being too dependent and too needy to get that one particular girl. I can tell from my own experience, it is a very hard battle to fight man, especially if you are aiming to be her BF...
If you are not ready to act fast as a lover, you will miss the first window. Once you miss the first window (which could be the very first day you meet her) and you are still after her, she will realize that you are a potential BF... Which means, that she will most likely make you WAIT because she smells the new BF frame... Think about it, she has a BF, a good guy in her eyes, that she is happy with, at least up to some point... She has nothing to lose at this time... If a new guy (you) comes around and shows interest, she will want to test you, just to make sure that you are a good replacement for him... IF you fail the first window, she'll know that you are not the fast lover who can bed her fast - and she will make it much harder on you should you stay around...
So first, you should develop lifestyle when you don't NEED that girl, which basically means developing Abundance Mentality. You should always have 3-5 girls sort of available to you, because if you get rejected by the one you really want you could fairly easily switch to another girl, while not feeling much pain. If you suffer One-itis and don't have other girls to focus on, you will be investing a lot into a girl that is taken, and the chances are HIGH that she will reject you because, well, because you are TOO NEEDY to have her... She will see you as a guy who really wants her but who has no other choices, no other girls available to you... That's no good... Remember, you can't be needy, it will turn against you, she will dump you before you even have a chance, and you can be hurt a lot...
So be ready for rejection, deal with it BEFORE you invest anything into that girl. Rejection can come in different forms. The worst ones are when everything goes well, you are making great effort, your hope is high, you think you have really good chances - and then it comes. She dumps you, all your effort wasn't rewarded, you don't have any other girl to focus on... You will be screwed, you will be hurt and the pain is real...
Second, like I mentioned, it is an uphill battle. You see a girl that you really like, you really want her, there are great vibes... but then she walks away and goes home - to fuck with her BF. That could also be quite painful, depending on how you view sexuality, what you expect from relationships and how you view monogamy. Basically you are investing a lot into a girl that is fucking somebody else, while you are not having ANY results with her...
With that being said, girls do go for guys who have high(er) value. Basically she will compare you with her current BF, and you you prove yourself that you have HIGH(ER) value than he is, she will eventually chose to be with you... Which is a good news.
You need to develop great vibes with that girl, good emotional connection. Emotions are not only positive, but also negative. A good emotional connection means that she needs to experience variety of emotions (not only positive) with you, while she keeps perceiving you as POSITIVE guy. Does it make sense? You need to be positive, she needs to love being around you. For example, you need to project that you are a dominant man, but you also need to project some weakness. You need to project that you have a great vision for your future and nothing will stop you from achieving it, while you could get easily pissed at something that stands in your way... Sort of up and down, back and forth, hot and cold... You get the picture...
Overall, she needs to see you as dominant and confident man, who has a great life to live (so she can join you). When she compares you to her BF, you should be exciting guy who can give her great emotions and experience, while she should start seeing her BF as someone who is boring, and low(er) quality man... Ideally she should realize that you have much greater value than he does. There are some PUA who emphasize her current BF weaknesses, but IMO you are lowering yourself down by doing this... You simply don't talk crap about other guys, you never put any other man down, make it a rule...
Then you will most likely need patience. Lots of patience, the reality is that she will not jump on you the minute you show some dominance... You have to be open minded. She may offer you window (sex) the first time you meet her. Or she could put her breaks on, which means that she will make you wait loooong days, if not looong weeks... Long weeks of you waiting and playing closely with the fire of painful rejection... But remember, being persistant, going after that girl for long time (while NOT projecting any neediness) is very powerful tool... She will perceive you as dominant man who goes after what he wants without giving up...
You need to remain emotionally cool no matter what. Be non-reactive when she talks about her BF (and remember, she will, she may just want to test you how are you going to react). She may go so far to tell you that she is fucking him, that he has a big dick and so on. If you react negatively, if you over-react, if you show weakness - you are gone. Burning rejection, all investment was wasted.... If you don't react, she will perceive you as cool and interesting guy who deserves more attention...
There is more to it, but in essence it is the same you read on GC over and over. You are the man, the cool guy who is perfectly capable of leading her - while she may not be complying at first because she is with another guy... You have to have strong frame, frame that "pulls her in". She needs to gravitate towards you - you are not chasing her but she is coming back to you because she really really finds you cool and interesting... Does it make sense? You are simply projecting a great value as a man, while you are not getting any results and only waiting, and waiting... It could happen today, it could happen 3 months from now, or it may not happen ever... The waiting is nerve wrecking, the question is - does it worth it? In the mean time while you are waiting, you will be mostly frustrated because you do most likely suffer of disease called One-itis... yet if you show any frustration and neediness to her, you are gone......
She needs to know that you are interested in her, without you chasing her... You are simply there, having great time with her while she is around, and you let her leave (to her BF) while remaining totally cool...
So do your best bro... Hope for the best but be ready for the worst. Remain COOL, PATIENT AND NON-REACTIVE no matter what happens, no matter how much time it takes - that is the key...