After cutting a piece from a to tall cactus, how do I heal the original cactus where the cut was made made or do I just leave it alone to heal
If you have already made the cut it probably doesn't matter. The cut should heal over in time. If you still have to make the cut then it would be best to treat the cut with a light dusting of charcoal powder or flower of sulfur powder. This is especially true if the cactus is of a kind that exudes a lot of sap. You can buy these products in a plant nursery.