Measure the basement space available. Check out heating and cooling duct work, vents, electricity and plumbing. Design a bathroom area at one side. Draw a workable plan for the game room with closets, shelving and a wall for media equipment. Consider including a wet bar.
Address moisture and mold problems before construction begins. Call a professional to help you identify and correct any problems. Don't begin framing the space or doing anything else until all moisture issues have been resolved.
Frame the walls. Insulate exterior walls between the framing studs. Staple plastic over the insulation for a vapor barrier. Run plumbing, vents and electrical outlets before you cover the walls with the vapor barrier. Install drywall over the framing.
Put in cabinets, light fixtures and flooring. Install tile flooring for easy maintenance. Buy light fixtures and furniture that can take heavy use. If possible, choose bright fabrics to make the room look like it's fun to be in and to compensate for any lack of sunlight.
Paint the stairwell and enhance the access steps. Consider covering the steps with carpet or adding textured porcelain tile. Make sure the stairs are not slippery. Add framed posters and prints to the stairwell.