The first thing is the gas fire place and there is normally wasted space above the gas fire place. If it already exists get the specs on your gas fire place model to determine your clearances. Knowing your clearances to combustibles you can rough in the opening to the unused cavity above the fire place.
The cavity space is usually 48 inches in width and 36 inches in height. Now you got this far you will need to factor in not only room for the T.V. but the components that go with it. You can add a recess on each side or floating shelves also work to house your components. Having your final design you can rough in your electrical, speaker wires, and all your specialty cords. Your specialty cords might need to be replaced so tubing in your wall from point A to point B will allow you to fish in replacement cords easily in the future.
Cabinetry is the next step. The best way to go is to design units that insert into to your framed rough in openings. Think about what wood and finish you want and also casings, mantel, doors with glass, or what ever your design requires. While your cabinets are being built you can repair the drywall, prim coat and paint two coats, maybe a new feature wall color.
Now the cabinetry can go in. Reinforce some backing to hold the weight if you are mounting the flat screen T.V. Finish your plug receptacles and plates. A good tip is buy vinyl blank covers so you can easily drill a hole to the size you need to pass your cords through. You are now ready to have your friends over and enjoy your new custom feature entertainment wall.