Holy water is made from sal salis. To learn how to make this, please refer to my article on How To: Create Alchemical Planetary Tinctures. It is used in consecration, purification, and in rituals. This is a six-step process once the sal salis has been created.
Place a glass of spring or filtered rainwater on your altar. Put some sal salis and caput mortuum seperated in a small dish on the altar. (Check back later for an article on altar creation).
Perform the lesser banishing ritual of the pentagram.
Face north with the altar in front of you, and extend your right hand over the salt, visualizing bright, glorious white light of holy energy flowing from the palm into the salt. Recite the following:
“So shall this salt of the Earth, purified by fire, become a heavenly salt which possesses the wisdom of creation. It will sustain my soul, spirit, and body, preserving them from corruption. It will nourish the Earth and fortify my heart. So more it be (or) amen.”
Extending your hand over the caput mortuum, visualizing the same light flowing from the palm. Recite the following:
“May this ash return to the eternal fount of living. So shall it bring forth fertile earth, bringing thus the tree of life. So mote it be (or) amen.”
Walk around the altar until you are on the east side, facing west. Recite the following:
“In the salt of eternal wisdom, in the water of preparation, in the ash whence the new earth springeth, be all things accomplished unto the age of ages. So mote it be (or) amen.”
Then, as you would creating a normal tincture (again, refer to my article How To: Create Alchemical Planetary Tinctures), mix your ingredients, filter, and store your holy water in a sealed, airtight container. Return the ash to the Earth.
Source: Secrets of High Magick by F. Mellville