If you have changed the clutch line, clutch master cylinder or clutch slave cylinder in your car, chances are you have introduced air into the clutch system. Air in the clutch system causes hard shifting or no ability to shift gears because the air in the fluid will compress, and the clutch won't disengage. To ensure that all the transmission components work together properly, you must bleed the clutch, a process similar to bleeding brakes.
Jack up the car and place on jack stands.
Attach a clear rubber tube to the clutch slave cylinder bleeder.
Push down on the clutch pedal and, while pushing down, count, "1, 2, 3." On 3 say, "holding." This will inform your assistant when the clutch pedal is fully depressed.
Have your assistant use an 8-mm wrench to open the brake bleeder while the clutch pedal is being depressed, at count 1. Close the bleeder two seconds after the clutch pedal is fully depressed.
Repeat Steps 2 and 3 until the assistant doesn't see any bubbles of air going through the clear rubber hose. At that point you should feel that the clutch pedal is a little harder to press down than before.
Lower the car to the ground and test the clutch system for proper operation. If the clutch remains "mushy" or "soft," you may have to repeat the entire process again until all the air has exited the system.
You can purchase a one-way bleeder that you can install on the clutch slave cylinder. The one-way bleeder acts like a check valve and will only allow air out, so that you can bleed the clutch by yourself without fear of introducing air into the system.
Wear safety glasses when bleeding the clutch.