When you are trying to make a girl look like a boy, you need to focus on camouflaging the areas of the girl's face that are feminine, such as dark or full eyelashes, cheek contours and fuller lips. Boys usually have a stronger jawline and a more prominent chin, but since these features are harder to create with makeup, you would have to bring them out and play down the contours of the face. In essence, it's the opposite of what you would normally do when applying makeup.
Fill out your eyebrows with a freshly-sharpened eyebrow pencil. It needs to be sharp enough to create very fine lines and needs to be the same color as your hair. Draw along the bottom edge of your eyebrows in short, light, upward strokes that mimic the way your eyebrow hairs are growing. Men's eyebrows tend to be thicker than women's, so keep this in mind as you fill in your brows.
Apply concealer that matches your skin tone to the top and bottom edges of your lips, and blend in to your skin. Only use a pea-sized amount or less--you don't want it to actually look like you are wearing makeup. You are only putting it on the edges of your lips to make your lips thinner and less feminine. Refrain from applying concealer or foundation anywhere else on your face.
Dab and brush your jaw and around your mouth with a damp sponge with dark eyeshadow on it to make you look you have stubble or a "five o'clock shadow." To make it look like the hairs are longer, lightly dab on some brown or black mascara to give yourself a mustache. Use a color for the eyeshadow or mascara that closely matches the color of your hair. If you are blond, pick a lighter brown or beige color. If you are a redhead, choose a color with a undertone. Go slow so that you don't overdo it.