Use a measuring tape to define the dimensions of the porch. Draw the house and the porch footprint on a sketch pad so you can plan the exterior and interior changes. Design an enclosure that allows at least one or two windows. Plan a wall or two of windows by using glass or screen panels, depending on how you want to use the space.
Take down old screening material to reveal the basic room shape. Install enclosure framing in metal or wood. Buy 2-by-4-inch boards to build exterior wall studs, for example, that will support basic house siding materials. Use an electric screwdriver to install any metal framing needed for sliding windows or screens.
Run new wiring for light fixtures and needed electrical outlets. Be sure to plan these electrical components away from any wall areas that might have moisture issues. Route electrical wiring through ceiling areas of the room into the home's attic and back to the house breaker box, as one option.
Finish up details for the interior. Install interior insulation before adding beadboard paneling or drywall to the room. Install wood trim and baseboards before painting the interior. Connect any light fixtures and outlets, which should all feature ground fault safety components in a three-season space.
Add flooring and furniture. Install tile over a concrete floor base or use indoor-outdoor carpeting as a good option for the porch space. Buy casual furniture and artwork for the room to give it an outdoors ambiance.