Bleeding a clutch slave cylinder is necessary to make sure all of the air is out of the hydraulic lines. Since air (even the driest air) still has moisture in it, you want to make sure that as much air as possible is out of the system. This is because the hydraulic clutch fluid---which is usually brake fluid---absorbs moisture. Once it does, it starts to break down and will not do its job properly. Change your clutch once every two years, or, if you live in a humid climate, once every year. Part of this process involves bleeding the clutch slave cylinder.
Place the box end wrench over the bleeder valve on the clutch slave cylinder. The exact location of the cylinder will vary depending on the make and model of your vehicle. The cylinder will usually be a long cylindrical component with hard steel lines from the clutch master cylinder running through it. The valve itself looks like a small screw with a hole in the center of it sticking out of the side of the cylinder. Orient the wrench so that it fits snug over the nut at the base of the screw.
Slide the clear plastic tubing over the end of the bleeder valve. Place the other end of the tubing into a catch pan or jar.
Have an assistant press on the clutch and hold in down to the floor. The clutch pedal should remain pressed to the floor.
Open the bleeder valve to release the pressure in the clutch fluid lines. Make sure that your assistant does not release the pedal pressure on the clutch. It needs to stay pressed onto the floor; otherwise, air will be drawn into the line. You will see fluid and air bubbles rushing out of the valve through the plastic tubing.
Close the valve by turning it clockwise until it is tight.
Repeat steps 3, 4, and 5 until the lines have no more air in them. Check the clutch master cylinder reservoir levels periodically to ensure the clutch fluid does not drop below the lower or "min" line on the reservoir tank.
