The Roman Catholic way of making holy water is as follows;
1. Place a handful of sea salt on sterilized glass
2. Make the sign of the cross or raised palm of benediction over the salt and recite the exorcism of salt;
" God's creature, salt of the Earth, I cast out the echo of evil by the living God, by the one true God, by the holy God, through whom all things have their being. May you be a purified, living, heavenly salt, a means of health for those who believe, a medicine for body and soul for all who make use of you. May all evil fancies be driven far from the place where you are sprinkled. And let every unclean spirit be repulsed by the power of almighty God."
3. Recite the blessing of salt;
"Almighty, everlasting God, I humbly appeal to your mercy and goodness to bless this creature, salt, that you have given for humankind's use. May all who use it find in it a remedy for body and mind. And may everything that it touches be feed from uncleanness and any influence of evil through your holy name, Amen"
4. Make the sing of the cross or raised palm of benediction over sterilized glass bowl of fresh spring water and repeat the exorcism of salt, exchanging the word salt for water.
5. Recite the blessing of water;
"O God, who for man's welfare established the most wonderful mysteries in the substance of water, hearken to this prayer, and pour forth your blessing on this element. May this creature of yours, when used in your mysteries and endowed with your grace, serve to cast out demons and to banish disease. May everything that this water touches delivered from all that is unclean and hurtful; through your holy name. Amen."
6. Now pour the salt into the water in the form of a cross, saying;
"May this heavenly salt and water unite in harmony, in the name of the Almighty God, Amen."
The Magic Shield - a Manual of defense against the Dark Arts.
Oh and anyone can make it, you don't have to be a priest or anything. You just need God's love aka his protection.
Also you can use it to banish a demon or protect your home by creating a barrier with it around the outside of your house.