Place butter over low-medium heat until fully melted. Add the bag of mini marshmallows to the melted butter and stir using a ruber spatula until the marshmallows completely dissolve.
Add rice crispy cereal and mix until fully coated.
Immediately add this mixture to the bottom of a 9x13 baking dish.
Using a sheet of parchment paper, press down on the rice crispy flatly with the palm of your hand. Press into the corners and sides of the dish until the rice crispy treat is as flat and flush with the pan as possible. Cover and set aside to cool to room temperature for about 30 minutes.
When done, carefully cut the rice crispy treats into rectangles using a knife. Make one cut down the center from handle to handle and 6 cuts along the side to make (14) total rulers.
Using the black writing gel, begin to make your ruler markings starting with the longest lines. Draw (3) lines along the long side of the rice cripy treat to the center of the rectangle.
Next, in between the edges and longest lines, draw (4) lines about half the size of the line you drew previously, parallel.
Fill in the remaining line spots with smaller lines, about half the size of the line you drew previously. Above the longest ruler lines, write out any number sequence of your choosing.