Cut a pizza box to the size you'd like for your canvas. You can use the bottom and the top of the box if you cut carefully. If you are really resourceful, consider using the sides and scraps as tiny canvases to create miniature art.
Paint over the logo of a pizza box if you are using it for painting. Paint in white or whatever color would work for your painting. You could also glue or tape blank white paper over the logo. Do what works best for you. It's up to you to determine the surface material that best compliments your art!
Give the box a second layer of paint after the first one has dried. This will not always be needed, but it is often needed when painting with light colors. You want a smooth, solid color canvas.
Paint on your canvas. Use your imagination, and create beautiful art from your heart.
Take your pizza box to any frame shop, and they will marvel at your creativity. You can have your pizza box canvas art framed, and nobody will ever know its origins--unless you tell them with pride about your creation!