Mark out measurements on the lumber. The treads should be 36 to 48 inches wide and the depth should be 10 inches. Use a measuring tape, straightedge and pencil to mark out the dimensions.
Cut through the lumber over the pencil marks with a circular saw to create the treads.
Apply wood glue to the back of each of the edges of the treads.
Set the treads onto the stringers; the edges of the treads should rest on top of the stringers. Press on the edges of the treads to create a bond between the glue, the tread and the stringer.
Predrill two holes -- one at the top and one at the bottom -- through the edges of the treads and the stringers.
Secure the tread to the stringer by drilling 3-inch deck screws through the predrilled holes.