Round each quarter with a knife, starting from the top moving down until you attain a round leg.

Bench nail seat

 beautiful handcrafted outdoor bench designs marble

He measures the mortises and carves corresponding pegs, or tenons, into the seat and back slabs’ ends. Using a chainsaw or wood chisel, cut a half-moon shape into the top of each bench base to provide a saddle for the bench seat.

Nail the backrest support logs to the end of each bench base using two nails on each side.

If you are using the traditional way, using the axe, place a wedge in the center on the marked line in the four corners of the log and in the center. For a best fit, the holes must not be straight but angled at 10 degrees from vertical away from the axis of the bench. For the bench base, you need four pieces (two for each side) of full logs, about 2-feet long. Nail two landscaping spikes or barn nails a few inches from each edge of the bench seat into the top of the bench base on each side.

Source: https://www.pinterest.com/explore/bench-designs/