Wooden decks are generally constructed of boards with narrow gaps between them, designed to allow rainwater to leak through. This method prevents standing water from rotting the wood, but it also means that a second-story deck makes for poor shelter during a rainstorm. If you want to be able to use the space under your deck during rainy weather, installing corrugated roofing panels to the deck's underside can solve the problem.
Determine the desired slope of the panels under your deck. For a house roof the recommended minimum slope is 2.5 vertical inches for every horizontal foot, but a nearly-flat roof is fine for under-deck installations.
Install a wood support structure for your panels, screwing a row of 2-by-4s running parallel to your wall every 2 feet along the underside of your deck. Trim the 2-by-4s to make them thinner or screw an additional row to the underside of the first, making it thicker, as necessary to get the slope you want from row to row.
Have an assistant hold the first corrugated panel in place against the rows of 2-by-4s on the upwind, house side of the deck. Use washered wood screws to secure the panel, driving a screw every foot along each row.
Position the next panel further from the house but overlapping the first by at least 6 inches. Secure it in the same manner as the first. Install subsequent rows of panels, working your way to the downwind side of the deck's lower surface, until finished.