First, convince your entire family to start collecting plastic bottle caps. To make things even more fun, don't tell them why they are collecting caps.
Select a suitable picture, in my case I selected a 30 x 36 image of Princess Peach. Be sure to select a small image as you're going to need one cap for every pixel.
Once you have collected at least 1080 caps, you're ready to start phase two.
(Note: It's been pointed out that the image is actually 28 x 35 totalling 980 caps. I forgot that I lost a row and and two columns when I planned out the border. Small oversight on my part, can't believe someone actually counted!)