My rough inspiration came from "Young Mad Scientist's First Alphabet Blocks" by Xylocopa http://www.xylocopa.com/product/mad-science-alphabet-blocks. As a science teacher I still plan on making a more science based set one day, but my graphic design skills need work.
Step 1: Designing the blocks
In the computer program Paint, create a template of large boxes for the letters etc. to go into. Two of the sides will be light (unburned) images on a dark background and two sides will be dark images on an unburned background with a dark border. Using copy and paste I created a 5 by 6 block pattern with equal spacing horizontally and vertically to allow for the edges of the blocks to eventually be rounded.
The actual size of the boxes is not important at this point. More important is getting maximum detail without getting pixilated.
With the templates created, copy and paste images to fit in each box, saving the files as separate versions. I used:
Side 1- Dark box with light capital letters (A-Z plus additional A)
Side 2- Light box with dark numbers (two sets of 0-9 plus basic operations)
Side 3- Dark box with light lower case letters
Side 4- Image relating to each letter. I used superheroes and cartoon characters. Maybe sports teams? Animals? Food?
Note: The hardest part of this project was finding or creating and then manipulating appropriate images. Ideally they need to be only black and white so that the burning process is an all or nothing operation instead of shading. Original artwork is preferred as it avoids any potential copyright issues.