First, you need to determine the length and width of your ladder. Since this ladder was going into the greenhouse, it's a little wider than I'd recommend for other uses. The overall dimensions of this ladder are 8' x 2', therefore 2 - 8 ft long 1 x 3 inch Spruce Pine Boards were used as the side rails.
In addition, 3 - 1 x 1 x 6 dowel rods were need to allow for 7 rungs that were 2 feet wide each. The only other supplies needs are wood glue, stain and/or paint, depending on the look you prefer for your rustic ladder.