Place the tree stump on its side. With the blunt end of a splitting maul, tap a splitting wedge at a 45-degree angle into one edge at the end of the trunk with the point toward the center of the trunk. Continue tapping to split the trunk in half.
Insert a second wedge in the split of the trunk when the crack becomes big enough. Strike the second wedge to widen the split. When the first wedge can be removed, insert it into the leading point of the crack and strike it. Continue alternating wedges until the trunk is split in two. Choose one half for the bench seat.
Chop off the bark of one half of the trunk with a hatchet. This step is optional. You may elect to leave the bark on for a more rustic look to your bench. If the trunk is thicker at one end, chop layers from around the thick end using the hatchet so the entire thing is of more or less equal thickness.
Place the trunk half on a flat surface with the flat side down. Mark a point on the curved side of the trunk half at each corner 2 or 3 inches from the edges. These are the leg hole marks. Drill a 1-inch hole at each mark all the way through the trunk. The holes should angle slightly toward the center.
Split four legs from the other half of the trunk. These will be the legs. Trim them to roughly 3-by-3 inches thick using the hatchet. Whittle one end of each leg to a 1-inch diameter that will fit tightly into the leg holes. The 1-inch segment should be slightly longer than the leg holes. Saw a slot in each leg on the 1-inch end across its diameter and 4 inches deep.
Drive each leg into a leg hole with a mallet so that it protrudes out the flat face of the trunk. The sawed slots probably will be jammed closed. Reopen them with the saw on the flat side of the bench. Drive a wooden wedge, 1 inch wide, 6 inches long, and 1/16 inch thick on thickest end into the slot on each with a mallet. Drive them as far as possible to make the legs as tight as possible in the holes.
Saw off the leg parts that protrude past the surface of the bench seat. Trim the legs to the same length if necessary. Sand the flat surface smooth.