Not written by me - from this website - wikiHow. Or something. Sorry.

Serena Van Der Woodsen is the blonde beauty characterized in the popular Gossip Girl novels. Shes described as extremely attractive to men with a sheet of golden blonde hair and large, lake-blue eyes. Though it is heavily stressed throughout the series that Serena is "one of a kind" and "the most gorgeous girl on the planet", its time to break those rules and step into her shoes for a day, a year or a lifetime!

Ignore malicious rumours spread by your cruel classmates. Serena wasn't fazed by the constant gossip and controversy over her alleged drug usage and promiscuous ways. Simply turn a deaf ear to your classmates when they decide to spotlight your negatives (because secretly, you know you're flawless!). After all, all this trash they talk about you is just making you famous! Granted, Serena has a rep among the Upper East Side as Blair's slutty other half, but hey! We all know her name now, don't we?
You do not need to spend a fortune on clothes! Yes, Serena is blessed with great financial wealth and therefore can afford Fifth Avenue name-brands, but remember, it's not what label you wear but how you wear it and in general how it looks! Get creative! Serena is famous for her style of "new clothes looking perfectly worn and thoughtless." Sift through vintage shops or scour the city for obscure hole-in-the-wall designers that sell avant garde outfits for bargain prices!
The less make up, the better! You don't hear about Serena caking on face powder, eyeliner and concealers! Serena wears the bare minimal of make up, just a touch of sparkly lip gloss (Urban Decay lip gloss in Gash, Chapsticks in Cherry and pale pink Vincent Longo gloss have all been mentioned). If you have problem skin, DO NOT FRET! Visit the dermatologist stat to clear up any issues. Always moisturize with non-scented creams (i.e Nivea) and don't go overboard on the face washing!
Have fun! If there's one thing Serena rocks at, it's partying! Totally enjoy urself, dance on tabletops , flirt with cute guys and dance your kilos off.
Try to develop Serena's awesome personality. Serena is super sweet to everybody. She never brushes people away, even if they are annoying. With this character all you need to do is smile dazzingly at every guy, and they will all fall in love with you guaranteed.
Serena is super confident which is why people are attracted to her. This doesn't mean bragging. This means not going "Oh my god, I'm such a loser I can't DO THIS". Let's say you throw a ball really badly and it hits someone else, don't say "I'm TERRIBLE AT THIS GAME OH GOD I SUCK!". That's bad. Say something more like, "Oops! Sorry!" and just laugh it off. People will like someone who is comfortable in their own skin, especially guys. Confidence is key if you want to be like Serena.
Create trademark features for yourself. Serena has her own trademark perfume, and a personal quote that goes with her signature: 'You know you love me.'
If someone makes fun of you for something that you have or like, pretend as if it's not a big deal. Snarky remarks aren't necessary. Be more like "Whatever. I still like it." instead of "It's not like your opinion matters to me." The second comment will not help the nice-ness factor you want.

Make yourself seen in classy boutiques and shopping complexes, and don't be afraid to go by yourself, although having friends with you help.
Find a best friend who is like Blair Waldorf: glamourous, smart, pretty and RICH (don't forget rich)
Don't get a fixed boyfriend. Serena mingles with everybody,so don't committ yourself to only one person!
Be bold be daring and sometimes be BAD. We all know S isn't the best-behaved girl in the Upper East. She has a mysterious history, so create your own.
READ THE GOSSIP GIRL BOOKS. That's the only way to know the characters! Also, don't forget to watch Gossip Girl on T.V.

People are going to spread the most ridiculous rumours about you, but just pretend you don't know about them.
Random creeps are going to perve on you, or possibly stalk you. Don't do anything about it, but avoid all trouble if possible.

Things You'll Need
The Gossip Girl books
Some money so you can buy some make up and clothes to make you look gorgeous.

I found this on a website but I think it's great! I don't want to sound cheesy but I do think Serena is a great role-model (you know, now, post-returning to the UES!). I've also never read the books so I don't know how it applies.