Install Lower Horizontal Supports

What happens here

Select an appropriate length of lumber for the lower horizontal support (e.g., a tread or riser blocking). Position and secure it between the two stringer sections at the lowest level. This piece helps to brace the stringers and defines the first step. Though not explicitly shown, use a level to ensure this piece is perfectly horizontal and a framing square to check its angle against the stringers to maintain squareness within the frame.

Goal: The first horizontal support is installed at the bottom of the stair frame, providing initial stability and defining the first riser/tread level.