Measure the opening underneath your stairs using a measuring tape. Add an inch to each side to allow room for seams.
Cut the fabric of your choice to match your measurements using a pair of scissors. Fold each edge over itself 1/2 inch and secure using sewing pins.
Stitch the folded edges using a needle and thread or loaded sewing machine. This will hem the curtain and ensure the edges don't unravel over time.
Attach clip-on curtain rings across the top of the curtain, every 3 inches or so.
Loop the curtain rings through an adjustable curtain rod.
Install the adjustable curtain rod across the top of the opening beneath the stairs, twisting the rod until it fits snugly.