Turn both cabinets upside down, and set one of the boards on top of them. Slide the cabinets to the ends of the board until the edges of the cabinet are flush with the edge of the board.
Predrill one hole down through the board into each corner of each cabinet. Fasten the board to the cabinets with the 1-inch wood screws.
Flip the assembled cabinets over, and repeat the process in Steps 1 and 2 for the other board.
Position one of the two shelf boards where you wish it rest between the two cabinets. Mark its location with a line on one of the cabinet walls; use a pencil to draw the line. Remove the shelf board, and drill two evenly spaced holes through that line on the cabinet wall into the cabinet.
Reposition the shelf, and use a pencil or long screw to mark where you should create holes on the side of the shelf that align with those in the cabinet walls. Predrill the two holes in the side of the shelf at the location of the markings. Attach the shelf in place using two wood screws, coming from the interior of the cabinet.
Set the level on the shelf board, and adjust the unattached side of the shelf until the shelf is level left-to-right and front-to-back. Draw a long pencil line on the cabinet wall to mark the location of the shelf's unattached side. Predrill two evenly spaced holes through that line, then move the shelf into position, and predrill holes into the side of the shelf through those holes in the cabinet wall. Attach the shelf in place with two wood screws, coming from the interior of the cabinet.
Stretch fabric along the back of the open section of the entertainment center, and staple it in place along one side. Stretch the fabric tight, and continue to staple the fabric in a circle. Leave one corner or section unstapled in each shelving compartment, so you can run electric cords through it.