Clear a workspace that is open and well-ventilated. Give yourself plenty of room to work on all sides of the bookshelf as it lays flat. If you decide to work inside your home, use an old sheet or drop cloth (which can be purchased at any home improvement store) to protect your floors and the finished product.
Measure the existing bookshelf. Get the dimensions from top to bottom, side to side and front to back. The piece described here is 36 inches tall, 36 inches wide and 11 inches deep, with four shelves.
Working with the bookshelf on the drop cloth, remove all the shelves except the top shelf, as your mantel. Some older bookshelves' nails or screws could be rusted or hard to get a pry bar around. Be careful when removing these, as you do not want to damage the remaining wood. You can always fill in the screw holes with wood putty, but you don't want the finished product to look like Swiss cheese.
Add a backboard if your bookshelf doesn't have one already attached. To do this, lay the bookshelf on the drop cloth, with the front of the shelf down. Secure a ¼-inch-thick piece of plywood that matches the height and width measurements--in this case 36 inches wide and 36 inches high--to the back of the bookshelf. This secures the structure to keep it from splitting apart. Secure the backboard using ¾-inch nails around the perimeter, being sure that the nails go straight into the frame of the shelf. Use one nail every three to four inches for strength and stability.
Cut three pieces of decorative molding to frame the front. Using a table saw if you have it (or a hand saw if not), cut the side pieces of molding at 45-degree angles for a snug fit at the top corners; the bottom corners need to be cut straight across. The side pieces in this example should be 36 inches long at the highest point of the angle: The piece on the right should be angled with the high point on the right, low on the left. The piece for the left should have the high point on the left, low on the right. Cut a third piece of molding to 36 inches on the longest side with 45-degree angles on both ends, so that it fits snugly with the side pieces.
Turn the shelf over onto its back, with the front of the shelf up. Nail each of the molding pieces onto the sides of the frame, with the longest sides being even with the frame of the bookshelf. Place a nail every four to five inches, and countersink each nail slightly. According to Miriam-Webster.com, countersinking means "to set the head of (as a screw) at or below the surface." To countersink, use either a pointed chisel or a larger nail and place the point on the top of the nail that has just been hammered in. Tap lightly to push the nail just slightly below the surface of the wood. This will give the fireplace a nice, clean edge.
Place the top molding along the top shelf so that it rests in the opening between the two side pieces, pushing it up close to those pieces. This also creates a finished edge along the mantel surface. Again, secure and countersink the nails every four to five inches.
Dab a tiny bit of wood putty into each of the nail holes on the front of the fireplace and let it dry for about 30 minutes.
After the wood putty has dried, use a fine-grain sandpaper and gently sand over the putty until the surface is smooth and even.
Paint the surface and interior of the fireplace. Standard latex wall paint will work well. If you want a more stylized look, use a high-gloss enamel in black for the interior of the fireplace and dark chocolate on the outside. No need to use high-heat paint--your faux fireplace will not be exposed to high heat.
Remove the paper backing from the self-adhesive felt strips and place on the bottom edge of the two side pieces. Be sure to trim the edges of the felt strip so they are not visible. This will protect the floor as you are moving it around, or if it accidentally gets bumped.
Set the faux fireplace in the location of your choosing and put a selection of candles on the floor inside it. Or add a fireplace grate to create a more finished look.