Launder and iron the sheet. A double sheet should fit a chair and a king-size sheet should fit most sofas.
Pull the chair away from the wall so you can easily move around it.
Drape the sheet over the chair, taking time to make sure it is evenly distributed at either side.
Push and tuck the sheet as deeply as possible into the crevices of the chair, ensuring that the fabric is pulled as taut as possible.
Fasten the sheet in place using T-pins placed in positions where they cannot be seen. T-pins are widely available at fabric, craft and home improvement stores. They will hold securely providing that you use them correctly. Holding the T-pin firmly at the top, push the pin halfway into the cushion in one direction, and then, with a flick of the wrist, as you would position a hat pin, push the pin in the opposite direction, until it has sunk into the cushion as far as it will go. It may take a few attempts to perfect this technique.
Pull excess fabric to the back of the chair. If the back of the chair is against a wall and will not be seen, simply gather the excess fabric together and secure with a rubber band. If the back of the chair will be in view, take the time to gather the fabric together neatly at the center of the back of the chair and secure it with a piece of ribbon in a coordinating color. Alternatively, you can arrange neat pleats in the excess fabric and use T-pins to hold it firmly at the base of the chair.