Measure and cut railroad ties to fit the perimeter of the bed.
Place the railroad ties on the ground, forming a boundary at the edges of the bed where you'll install them.
Score a line into the ground 2 inches away from the railroad ties with an edging shovel on the outer perimeter of the ties and on the inner perimeter. Score around the entire bed. Then move the ties out of the way.
Dig a 6-inch-deep trench around the bed between the two score lines.
Pour 2 inches of gravel into the trench and smooth it with a rake to make it level.
Drill a pilot hole 1/2 inch wide into the railroad ties, drilling one hole every 3 feet.
Place the railroad ties into the trench with the pilot holes going up and down.
Lay a level on top of the railroad ties. If necessary, place a wooden block on top of the ties and hammer them down to make them level.
Drive 2-foot-long sections of rebar through the pilot holes in the railroad ties into the ground with a sledgehammer until the rebar is even with the top of the ties.