First, tape up the side the opens well
Cut off the top part of the box with scissors
Add more tape around the sides and inside to secure any loose pieces
Cut a piece of straw an inch shorter than the length of your box and glue it down.
You might want to think for a minute what you want the design to look like and then cut and glue accordingly. Make sure the spaces are large enough for the marbles you'll be using.
When you're done, cut a small hole large enough for the marble at the start and finish of the maze. Using an exacto knife was useful here.
Grab you're adorable preschooler and show them how it works, making sure to tilt the box very gently so the marble will go through the maze correctly.