1. Find a flat, head on image of the design you want.
2. Open in Gimp
3. Scale image to appropriate size (1 pixel less than canvas size in paint tool). Image --> Scale Image
4. Set up your grid. Set the spacing to 1 pixel width and 1 pixel height. Select whatever color you want for the grid lines. Image --> Configure Grid
5. Enable the grid by checking show grid. View --> Show Grid
6. Probably your grid will be obscuring your image at this point because you're zoomed out (IE a black grid will make it look like the image is all black). All you need to do is zoom in, I use the navigation window throughout the whole process to zoom in and out, look at different parts of the image, ect. View --> Navigation Window
7. If you want to change the colors to make it easier to transfer into paint tool, you can do so in the image menu. Image --> Mode --> Indexed (select generate optimum palette and enter how many colors you want).
8. Now, in Paint Tool, use the smallest pencil or line size and transfer it over. So, for example, if your grid shows pixels 37, 26 through 40, 26 as being red, select the red color in the Paint Tool and put a red dot in 37, 26 through 40, 26 on the screen. Do this for every pixel so that each one is the same color in Paint Tool as it is on your computer screen.
This is what I've been using, but I bet there is more that could be done by more savvy Gimp/Photoshop people to make it even easier. This method is very simple, but potentially time consuming for more complex designs. I don't know how to print an image with the grid, and would be skeptical about whether or not that would work to well, since you have to be pretty zoomed in to see individual pictures.
Thanks, Ben. I'm gonna give this a try when I get a chance. I've been trying to find a good way to do image overlays with the grid, since it seemed a little off from the game. And thanks to everyone else for their advice/tutorials.
And as far as how to print with the grid, I guess you could just hit Print Screen when the grid is up and put it through GIMP and print it out if it's not too big. Might still be hard to make out the boxes tho.