Power off,
Manually put all the axis in their center position.
Power up,
Click Home All.
All axis's should move towards the endstops on their own slowly.
If any axis moves away from their intended endstop, shut down the machine and you will need to flip the homing direction in your firmware. (Sprinter or Marlin)
Each axis should stop as soon as they activate their respective endstop.
Depending on your firmware settings, it may even stop and change direction and then approach the end-stop even slower to be more precise.