Hold the skeleton key with the bits on the end of it sticking straight up. Insert the tip of the key into the lock and push half the key in. Feel the resistance that the tip of the key meets.
Turn the key to the right. You may hear the lock mechanism clicking open. Alternatively, you may need to adjust the position of the key in the lock once more so the tip of the key correctly touches the correct parts of the internal lock.
Twist the key all the way to the right, completing a half circle movement with the key. The lock will click open, allowing access to the cabinet.