Find A Girl Whose Boyfriend Doesn't Appreciate Them: This shouldn't be hard. Maybe try going to stand outside Old Navy waiting for all the girls whose boyfriend's gave them Gift Cards for Valentine's Day.
Give Her A Valentine: Just pick one up at Walgreen's, candy boxes are like 75% off. Try and look kind of forlorn and dejected. When she approaches you and asks what's wrong say, "It's my girlfriend She died."
Cry A Little Bit: But not a lot. A single tear would be perfect. No sniffling. You don't want to look like a pussy. Then say, "When she died I vowed that I would give her Valentine to the next most beautiful girl I saw." That's when you hand over the chocolate and grab the new girl's hand. "She would have wanted it this way."
Embrace: Obviously, she's going to comfort you now, most likely in the form of a hug. The trick is to just hold the hug for 5 seconds longer than is normally socially acceptable. During this time to try emit as much sexual energy as you can. She will feel it, and as you pull away, your eyes will meet. She'll lean in, and your lips will meet.
Have Sex With The Girl You Just Met: Sneak out behind the Old Navy or whatever. She thinks she's helping you get over your dead girlfriend, so take advantage of that. For instance, if she rejects your desire to do it doggy style you say again, "She would have wanted it this way."