It only takes an hour or two from log to completed bench. Use a chainsaw, folding rule, set square, hammer, and nails – and do not be unecessarily careful, you will get a good result anyway!
Diagonally sawn 2”7 (51×178 mm):
two 185 cm, two 35 cm.
1 1/4”7 (32x178mm):
one 140 cm, four 35 cm.
Draw a curve on the 140 cm long board (e.g. by attaching a pencil to a 150 cm long string and fastening the other end of the string 142 cm from the centre of the board). Let the curve barely cover half the board width. Saw along the drawn line with a chainsaw.
Nail two of the legs (A) to the curved board. Make sure that the curved board does not protrude from the outer side of the leg board.
Mark the middle of the short, diagonally cut boards (B) and nail them to the legs, centred on the end of the curved board. Let the diagonally sawn piece stick up 3-4 mm above the leg board and the curved board.
Attach the two other legs with nails (C).
Cut of the leg boards with the chainsaw (C) so that they are in line (D) with the upper corners of the short diagonally cut boards (B) and the lower corners of the legs.
Place the long, diagonaly cut boards (E) on the construction, and nail them to the short, diagonally cut boards with a space of 1 cm between them.
Finally, you carve off the rough edges with a sharp knife.