If you own a spring-operated clock of any variety, you will need to wind it every once in a while to keep it ticking. An 8-day clock is a type that needs to be wound approximately once a week. There is a built-in grace period of one day, thus it is called an "8-day" rather than a "7-day" clock.
Open the front glass cover, if the clock has one.
Insert the key into one of the winding holes. There are usually two of these, one for the time and one for the strike. These are located next to the 4 and the 8 on the face of the clock. Some clocks have only one hole (for the time) and some have a third hole (for the chime).
Begin turning the key. Some 8-day clocks wind clockwise and others wind counter-clockwise. The direction may even be different for each hole on the same clock. If the key won't budge, then you know that you are trying to wind in the wrong direction.
Wind with a smooth, gentle motion. As you turn the key, you will hear the clicking of the teeth of the ratchet wheel. After each twist of your hand, let the key come gently back to rest on the last ratchet tooth before letting go and beginning the next twist.
Stop when you feel substantial resistance. You are done winding.
If you do not have a winding key or if you need to replace your existing one, ask a clock shop to measure the winding holes for you. In most cases, they will be able to sell you the right key on the spot or order it for you.
Make sure your key fits snugly in the winding hole. If it is too small, it could slip while you are winding and hurt your fingers. Get a key that fits. For the same reason, make sure your key is not cracked. You don't want it to snap while you are winding.