Locate a picture that you'd like to use as a stencil, either in digital form or in paper form. If the picture is in paper form from a magazine, book or photograph, then scan the photo into your PC. Save the scanned image as a .jpg or .gif so that it's easily imported into photo-editing software.
Start your photo-editing software. Import the digital image through the main menu options and center it on your workspace.
Remove the background form the image by selecting a brush tool and painting the background white. Create a new picture layer and then select any light areas in the picture that you wish to use for the stencil and paint them black so that they'll remain when you alter the picture.
Use the editor controls to adjust the brightness and contrast in the picture, creating stark differences between light and dark colors. Change the image to greyscale after creating the contrast, and then adjust the contrast and brightness even further by maximizing the settings to create a black and white image.
Adjust the image by connecting areas of black color so that all the black throughout the image is connected. This eliminates having to create multiple stencils to complete an image that has islands of missing stencil within it, such as eyeglasses that are missing the eyes because you didn't connect the black of the eye details with the eyeglasses.
Print the image onto a piece of card stock.
Place the card stock onto a cutting pad and then use a hobby knife to cut away only the sections of the image that you wish to paint onto a surface through the stencil.
Use the stencil by placing the card stock on your painting surface and then spraying through the cut portions with paint, creating an image with the blank spaces.