Determine the length of your bench, taking into consideration its location and whether it will be portable. Mark your wood plank this length, and draw a sawing line using a carpenter's square. Cut the plank along the line, using a circular power saw or a handsaw,
Prepare the legs of the bench. Pick a section of log from your unsplit firewood stash. The standard height for a chair or bench is right around 20 inches. Adjust this measurement as you wish. Subtract the width of your plank to determine the height for the legs. Your log height should match this measurement. Using your ax, split the log into two reasonably equal pieces.
On a flat surface, set the plank on the log legs. Each end of the plank should extend 6 inches beyond each leg. On the underside of the plank, lightly trace each leg outline where it meets the bench.
Turn the plank over and set it across sawhorses. Mark two drilling holes 5 inches apart, centered over each leg outline. Adjust the holes if you are using a very narrow or a very wide plank.
Drill a hole at each marking, completely through the plank, using a 1/4-inch diameter drill bit.
Replace the plank on the leg sections, matching the penciled outlines to the tops of the legs. Drill through the holes into the legs.
Check the level of the plank. If necessary, slip wood shims -- slender, wedge-shaped wood slices --between the underside of the plank and the leg section to bring the plank level. Leave shim in place. If it blocks a drilled hole, drill through the shim as well.
Secure the plank to the legs with lag bolts directly through the drilled holes. Use a crescent wrench to tighten the bolts so that the bench does not wobble. Recheck the level, and tighten bolts as necessary.
Sand the entire bench with an electric sander and medium-grade sandpaper. Pay special attention to the edges of the bench seat, to eliminate any splintering. Slightly round the corners of the plank.
Apply a stain containing waterproofing urethane with a brush or cloth. Allow to dry. Apply a second coat.