Thoroughly soak the key with a quality penetrating oil.
Place the puller over the head of the key with the three 12mm bolt heads facing you (as shown in the picture) and rotate the puller slightly to the left (clockwise) so that the key head is no longer aligned with the cut out notch, but is immediately under the nearest 12mm bolt.
You are now ready to apply pressure to the key head by winding in the three large bolts. You can place some packing steel between the three bolt ends and the hub of the flywheel to prevent marking.
Be patient. It is important to keep the puller parallel to the flywheel at all times by applying an even pressure to the key. An angled puller will apply uneven pressure to the key and increases the risk of shearing off the head.