What you need:
Long straightedge/ ruler
I used one big box for the body of the train, but you could use a few pieces and glue or tape them together. Then I measured where I thought the train should sit on the chair. I freehanded the design for the top of the train and used a straightedge to make bars, leaving a gap big enough for him to get his arms out and wheel himself. Once I cut out the train, I fit it onto the chair and marked where the brakes hit so I could cut out a spot for them. Then the fun part! I spray painted the inside of the train, and decorated the outside with acrylic craft paints (I tried kids' poster paint first but it would have taken 100 coats to cover) and glitter.
For the wheels, I measured and used a big compass to draw the right size circles. I cut them out, painted them, and affixed them with zip ties. The smaller circles were hard to attach, but I managed with a couple of zip ties and a few extra pieces of cardboard. The exact process is probably different for every chair, so you may have to figure that part out on your own. :)
So cute! Thank you Lauren for sharing your costume creation!