Buy the mug you are planning on printing on, and use it to measure the size of the image you will need. Take a picture of the image or images with a digital camera, and download them onto your computer. You can also scan a regular image or picture into your computer if necessary.

Open your photo in a graphics software program, and edit it to your preference. Most images are originally displayed in a rectangular formation, which may look strange on a mug. Images can be properly cropped using the selection tool and cropping command in your specific program. You can also add text to the image and add in different colors. Picasa is one of the most common choices for image manipulation software and often comes preloaded on new computers; however, any other graphics/photo program will work well.

Size your image properly before printing, using the print dialogue box to make the proper adjustments. Print the image on plain white paper to gauge results, and then when satisfied, print the image on the water slide decal paper. When printing on the water slide decal paper, be sure to print on the glossy side. Use white paper for colored mugs, and clear paper for plain, white mugs.

Spray the image with the fixative spray, which keeps the ink from running. Let the paper completely dry, then cut out the image and place it in water. The paper should eventually curl, and after it does, lift off the decal image.

Slide the decal onto the mug, pressing it lightly along the edges to secure it to the mug. Allow the mug to dry. Hand washing the printed mug instead of putting it in a dishwasher will prolong the life of the decal. You can also seal the mug with EnviroTex resin sealer, which will allow for washing in a dishwasher.