Paint by Numbers is an approach (with a kit) that allows people even with no knowledge about painting to paint all kinds of masterpieces by themselves.
Paint by Numbers divides a picture into sessions, each marked with a number that corresponds to a particular color.
Simply painting all outlined and numbered sessions with the corresponding numbered paint provided in the kit, and ultimately the picture emerges as a finished beautiful masterpiece, even if you've never painted before in your life!
Try to paint one color at a time and clean the brush before you start the next color.
How to Paint By Numbers?
The outlined and numbered sessions are normally printed on high-quality canvas (preferred! - as canvas last long), and the paints in however many colors you'll need as well as the paint brushes are always included in the Paint by Numbers Kit. 
In most cases, the canvas is already stretched over wooden stretcher bars. You can also choose to stretch yourself or frame the paintings yourself though.