Step 1: Gut the cabinet.
Some of the pieces you remove may be useful. Keep what you think you will need for this project, or another future project.
I saved the wooden piece that separates the TV from the Speaker enclosure. You'll see what I used it for later.
Step 2: Build the inside cabinetry.
This project took almost an entire sheet of plywood.
1) From the images in step 1 you can tell that the cabinet does not have a back. I measured the inside opening and cut a piece to fit (I will leave all the mwasurements off this instructable since all TV cabinets have different dimensions).
2) I nailed the new back piece to the two rear verticals.
3) The removal of the front speaker area revealed that I would need to raise the new cabinet "floor" a bit. I used some scrap lumber and made two pieces after I measured the depth and length between the front and rear verticals.
Step 3: Build the cabinet floor.
1) I cut this square shape out and notched the two front corners so I could snugly fit it beteen the verticals.
2) Nail the floor sides and front straight down into the base of cabinet. The rear will have to me nailed from the back.