Measure all of the garage space to determine the layout. Decide if you want to construct a media wall for a den or heavy-duty shelving if it will be an arts and crafts room. Go over the best way to route electrical wiring to the new room before construction begins. Bring wiring from attic space, a crawlspace or directly through an existing wall, for example.
Remove the garage door and frame the outside wall. Take down the garage door and remove all its tracking. Construct a solid wall where the door stood. Use 2-by-4-inch framing across the opening area on 16-inch centers. Be sure to allow an opening for one or more windows. Plan the garage-door space to conceal the fact this part of the house was a garage. Cover the exterior with siding to match the home's exterior and install energy-efficient windows.
Frame the interior of the space, including built-in wall units and any closet areas. Run electrical wiring while walls are open with drywall covering. Add fiberglass roll insulation on all exterior walls.
Cover the interior with drywall and fill gaps with joint compound, then sand it and paint the wall. Install crown molding around the ceiling and wood trim around a closet space.
Finish up room details and flooring. Hook up electrical outlets and light fixtures. Install tile, hardwood flooring or carpet. Add baseboard material and hang interior doors.