I began with an old tire, and found a small emergency spare tire that was in pretty good shape in a junk store for $5 US. I scrubbed the tire clean, then sprayed it with clear polyurethane so that the black rubber wouldn't get all over a person's pants when they sat on it.
Next I made a small stool slightly smaller than the diameter of the tire. I made the top from 1x4 pine lumber that I had left over from a previous project, and sawed the legs from some heavy duty 3 inch tubing I had salvaged from an old outdoor awning. Of course, the legs could be made from wood, or pipe, or whatever you wish. I covered the top with plastic, and drilled a 1/2 inch drain hole in the middle so that excess water would not accumulate.
I attached the tire to the wooden top with four wood screws, filled the tire with potting soil, then added enough grass plugs to fill out the seat.
My last step was to add an extension to my drip irrigation system so the stool would be automatically watered in the summer months.
This turned out to be a really comfortable stool! I can sit on the grass without having to worry about how I'm going to get up! I'm now trying to convince my wife that she needs to take care of mowing the stool.......