Buy a screen house kit. Screen House offers 4 different kits designed to transform your deck to an enclosed area. "The Polaria" and "The Gazalum" can stay up year round. All 4 kits can be assembled in 1 to 2 hours. Once your kit arrives, you need to install it based on the directions provided.
Build your own. Measure the deck and plan the basic structure. You need to attach posts at the corners of the deck to support the screen door panels. Depending on the width of the screen door panels, you should attach intermediate posts between the panels for additional support.
Attach the posts. The outside posts support the roof. Secure prefabricated lattice to the posts and the roof. Use a staple gun to put screening under the lattice to enclose the roof. You can slant the roof to improve aesthetics. Some people prefer to use a vinyl roof instead of lattice.
Screen the deck in. If your deck has stairs, install your screen door at the stairway. Otherwise, pick a convenient location for it. Attach the screen door paneling to the posts. You may want to use border trim along the top of the paneling for extra support and to provide a more finished look.