How can a boy pass himself off as a real girl?

It's quite simple really.

First, start small. Go out, buy some jeans (a 4 regular is the same as a guy's 28x29 at American eagle.) , maybe an A line skirt or two. No minis. Shave the legs. If you don't know much in the way of makeup, don't worry about it (have some gal friends help you out.). Really, all most of us go with is lipgloss, mascara, and some eyeliner. If your hair isn't long, let it grow out, and get it cut in a feminine style. Let your nails grow SLIGHTLY. Just barely past the edge of your finger will do. Cut them straight, and use a transparent polish if any. Go out, get some undergarments (when choosing a bra, light ones with light clothes, dark for dark, only get an A or B. that way it looks somewhat natural on you.), especially a Corsette. to maintain the feminine torso look, you must wear these at all times. Get some dresses, at least one of each of the following: a cocktail dress, a 'church' dress, and a sun dress. Make sure to have matching purses or clutches, shoes and earrings. Oh, yeah, you should get your ears pierced as well. Work on your posture (lower the shoulders and arc the back), the way you walk (swing your hips, but only slightly.), the way you talk (talk with your hands a little, get your voice pretty soft and higher pitched, but don't make it evident that you are trying.) Work out more with cardio related activities rather than weights. Cut the junk food. Only snack on veggies or fruit.

You may also find it useful to consult a stylist.