Figure out if there is a cabinet space large enough in your entertainment center for your subwoofer. You want to put the subwoofer on one of the lower shelves of the entertainment center to support its weight. Use a tape measure to get the measurements of the length and width of the subwoofer, than apply these to the measurements of a cabinet in your entertainment center to make sure it is deep enough and tall enough to hold the subwoofer.
Use the saw to remove a lower shelf so the subwoofer's height can fit, if necessary. Fit the subwoofer into the cabinet lengthwise or on its side, up and down.
Remove the subwoofer from its measured space. Use the saw to cut a small circle in the back of the entertainment center near the bottom. This is for the cords to go through from the subwoofer to the power unit, and to hook up to the television or stereo.
Use the sandpaper to sand down the edges of the cut wood if necessary. Paint or stain these parts of the entertainment center and the area where the subwoofer will go, if desired. Let dry.
Place the subwoofer in the cabinet you have created or modified. Create a shelf on top of it by sliding the plywood sheet on top of the subwoofer and trim it to fit. This creates a makeshift cabinet top for the subwoofer, so you can set things on it if desired. Or, you can take wooden boards the width of the cabinet, and secure them from the outside with the hammer and nails a couple inches above the top of the subwoofer. This creates a sturdier, more professional looking shelf.