Cut each end of the log flat on a horizontal band saw. If you do not have a horizontal band saw, you can use a chain saw. However, is it very important that the stump rest as straight up and down as possible when placed on its end. This will help to ensure the stool remains stable when you sit on it.
Trace a 16-by-24-inch rectangle onto the top of the log. Position the 16-inch edge at the front of the log. Again, make sure you use a log at least 4 feet long and 3 feet in diameter. The size of the log allows you to increase the size of the rectangle to 22-by-30 inches if you need a bigger seating area.
Grind the wood out of the rectangle with an angle grinder. Cut down 24 inches deep. Think of the rectangle as a cube. You must remove the cube of wood while keeping the back edge, side edges and bottom edge flat. Grind in sections to make it easier. Start at the front of the log and work your way down and back. When finished, the space left behind the rectangle serves as a the chair back and the material left on each side of the rectangle serves as the arm rests.
Smooth out sharp edges of the log by grinding a radius on the edges of the log with the angle grinder.
Paint three coats of clear polyurethane onto the seat and the bark. The coats of polyurethane will seal the wood and prevent it from cracking and rotting. It will also prevent the bark from peeling off. Apply the urethane with a paintbrush. Wait 24 hours between coats of sealer.