It can be somewhat difficult to achieve the clean lines of cel shading in traditional media unless you are working with actual animation tools, but it is still possible to use a cel shading technique with an ordinary drawing. Sketch an image, then outline it in ink. The ink lines must be fairly clean. Erase the sketch lines, then color in the image, using no shading. You could potentially use any media, but marker and acrylic paint both lend themselves well to this technique. Determine the light source, then shade in large, general areas and shapes with a slightly darker color without blending . If you are using marker or acrylic paint and allow the first layer to dry completely, you can more easily add a new layer with no blending.
Artists use a cel shading technique with digital paintings more often than with traditional media. Sketch an image using a digital program or scan in a hand-drawn image. Ink the image digitally; a clean line art is very important when cel shading digitally. Remove the sketch. Create a new layer underneath the lineart layer and color the image using flat colors with no shading. In most painting programs, you can highlight an area in the line art with the Magic Wand tool, then go to the new layer and fill in the highlighted area with the Bucket tool for fast colors. Determine the light source. Select a slightly darker shade of the color of an area that you are going to shade, choose a hard brush, then color in large, general areas and shapes on the image. You can add cel shaded highlights as well.
There are several ways to make a photo appear to be cel shaded. Select areas with similar coloration. You can do this using a Lasso tool, Polygonal Lasso or Magic Wand tool available in most digital painting programs. With the Magic Wand tool, you may need to change the settings so that it does not select too large or too small of an area. Fill in the selected area using the Bucket tool. This technique is best for photos that are relatively simple and have few colors, and it does not require a line art like other methods of cel shading. You could potentially use this technique on a digital painting as well.
The digital painting and manipulation program Photoshop has some filters that instantly create the appearance of cel shading. Open up a photo file. Duplicate the photo onto another layer so that you have two identical layers, then set the top layer to "Overlay" in the layers tab. Under "Filters," choose "Stylize" then "Find Edges." This creates a soft outline around similar colors in the image, making it seem cel shaded. You can also use "Artistic" then "Cutout" under "Filters." This flattens similar colors, removing subtle shading and making the subject seem cel shaded. Other programs may be able to create similar results, using filters with different names.