Trace the firewood piece out on to a scrap piece of 1/4" thick plywood.
The secret to the sturdiness of these stools is a hidden plywood piece under the firewood stump top. As the firewood piece dries, it will split, creating weakness in the stool top. The plywood will prevent the firewood piece from completely splitting appart.
Use a compound miter saw, set to 20 degrees off square, to cut the legs. You will need three legs per stool, each leg cut 12” long, long point to short point measurement, ends cut parallel at 20 degrees off square.
Three legs are recommended over four. The stool will not rock, even if the firewood piece is not cut perfectly square or warps over time.