::Unfinished wooden blocks, however many you choose- I got a set of 26 from ems woodbydesign on Etsy.
::6 images per block, cut to fit the size of your block face- my blocks were 1.5", so I cut my images to 1.25". I got my letter and number images from Leo Reynold's flickr page- he offers his images free for download and use under a Creative Commons' license. You could use magazine images, scrapbook paper, any kind of paper products as well.
::Mod Podge. I'm sure there are other mediums that would work just as well, but this is what I used.
::Scissors. Sharper the better.
::A foam brush. Other brushes would probably work too, but I like the foam brush because it leaves less of a brushstroke.