Step 1: Make cookie presentation background
To start, first lay out the 50" x 25" butcher paper sheet that will serve as the background. Use a ruler and pencil to lightly mark a 2.5" grid across the whole sheet. You're going to erase the guide lines later, so don't use a pen or draw too darkly. Sanity check to be sure you haven't mis-measured and that the grid is at least 18 squares across to be sure that your table will fit.
Once the grid is ready, using a pencil lightly outline the shape of the periodic table and each of the groups to make sure it will be correctly positioned. Once you've verified the layout, color in the groups with different colors of marker and outline the table grid with heavy black lines. After the final colors are in place, erase the pencil lines to create a finished look.
Select a separate color of paper for each periodic table group and and cut it into 2"x2" squares. The square will sit on top of the marker color so pick something that contrasts a little. Letter and number the paper squares to match the elements on the table, and after each tile is lettered glue it in position. The nice thing about doing the lettering on separate tiles is that if you mess up the lettering alignment you can do it over easily.
Once the colors and tiles are all complete, coat the whole table in contact paper to create a surface that will stand up to buttery cookies without getting stained with grease and chocolate. At first I wasn't sure that contact paper would be robust enough, but upon actually completing the project I found that contact paper (I bought paper labeled as "book covering" at an office supply store) is perfect. After it's been covered the whole table can stand up not only to holding cookies, but to being washed vigourously with a sponge with no ill effects.
The one catch is that contact paper strong and water resistant, but it isn't particularly sticky, meaning that it will tend to peel off at the edges. This is great when you're trying to align big sheets of contact paper over the table without getting wrinkles in it, but a real problem when you're trying to wash it. To deal with this, once the contact paper was in place square up the edges of the table and coat the edge with scotch tape folded onto both sides of the table. This creates a nice edge and prevents the contact paper from coming unstuck.