Dispose of or move the contents of the shed. Clean the interior walls and floor with soap and water. Use bleach to remove mold and mildew, and allow the shed to air and dry out.
Replace the door and reinforce the roof. Uninstall the original door and replace it with a new one equipped with a lock. Replace or install wood beams in the interior of the roof to give it added support. Replace or add shingles to the roof's exterior as needed.
Examine the flooring. If the flooring can be salvaged, restore the original flooring as this will give the space a sense of antiquity and imbue it with a vintage look. If not, replace it with a durable material.
Insulate the walls. Fasten 2-by-4-inch studs to the walls every 16 to 24 inches. Attach wall insulation in between the studs. Cover the newly added insulation with paneling or drywall.
Replace any existing windows with low-emissivity glass windows. These panes create a light barrier and help to insulate the structure. Caulk around the exterior of the windows to prevent rain from seeping inside.
Hire a licensed electrician to run electrical wiring to the shed and install several outlets. Ask for recommendations on wiring and safe load requirements. Because sheds are often made of wood, keeping the number of power outlets within and electrical demands placed upon the structure low helps to prevent electrical fires.
Engage a licensed plumbing contractor to run water and sewer lines to the shed. You may also consider adding a small sink and/or half-bath if there is enough space.
Paint the exterior and interior of the building. Select an interior paint of your choosing. The exterior paint should be acrylic latex to help prevent color fading; acrylic latex paint also better adheres to surfaces.