Purchase a converter kit, which pulls together two twin beds to easily create the size of a king-sized mattress (see Resources).
Lay out the twin bed frames and mattresses side by side in the king-sized bed frame.
Place the provided Velcro belt around the two twin-sized mattresses to hold them firmly together. Secure the provided buckle around the Velcro so the belt stays in place.
Position the plush center strip down the middle of the twin-sized mattresses, which will cover the crack to provide a seamless feeling to the mattresses. Attach the center strip to the Velcro belt to keep it in place.
Cover the mattresses with a pillow top cover if desired. Then, add a king-sized fitted sheet over the bed to create the appearance of a king-sized bed rather than two twin mattresses.
Make the bed with the top sheet, and cover with king-sized comforter. To further hide the twin-mattresses, place a bed skirt around the bottom of the mattresses.