All you have to do is import the image that you'd like to convert, choose the color pallet for the finished product, make a couple of adjustments (if necessary), and proceed with the conversion.
Once you have the image read you can choose whether to save it to your hard drive, or print it on the spot. (Note that in the trial version, you won't be able to save your images.)
Color By Number is a curious program that lets you play around a little with your photographs. It's nothing too spectacular or fun, but it does offer some entertainment.