Holy water is just water that has been properly blessed by a priest. There are thus no ingredients, other than the most common ingredient of all, and there's not a lot of time or steps necessary to make it, which is why the price on equipment lists is pretty low; that's really the price for the vial, plus a donation to the church. (Holy water is generally free to the members of the congregation, but roving mercenary-adventurers are outsiders...)
The real requirements for holy water are some kind of shrine and a font or basin to contain the water while it is blessed. However big the font is, that's how much holy water a priest can produce each day. The steps involved are:
- Get land for a shrine and sanctify it;
- Build an altar at the shrine and sanctify it;
- Sanctify a font or basin;
- Put the basin on the altar, fill with pure water, and sanctify it. Repeat this step when desired.
There may be a cost to Step 1, if no free land is available. There's certainly a cost to building a church, temple, or chapel on the land, but theoretically that's not needed; it's for the protection of the sacred items and regalia kept at the shrine, and for the protection of the congregation from the weather during worship. There's possibly a cost to Step 2, in which case size matters, but if a carpenter and a woodcutter volunteer and the wood can be acquired for free, the altar is free, too. The same applies to Step 3, although it's certainly more likely to involve a cost. Step 4 is free to the priest involved, unless somehow water isn't free. Removing the water from the basin, however, requires a blessed container unless the holy water is used immediately.
For the sanctification process, I'd use the Turn Undead roll. Clerics get to add double their level to the first roll of the day, but additional attempts by the same cleric or at the same shrine do not get the bonus. On a Good result (9+,) the land or item is sanctified; on a 2, the land or item is cursed. The priest does not know the result, but must act on faith.
The first three sanctification rituals must be done, but as long as one of the three works and none of the items is cursed, a priest can properly sanctify holy water at that shrine. If any of the three items is cursed, or if the sanctification of the water produces a curse, the water will curse things it touches or strengthen demons and the undead.
The font for holy water can be incorporated into the altar, which allows for larger quantities of water to be sanctified. However, smaller, portable basins are often used instead so that the basin can be removed to make space for other items to be sanctified on the altar. For example, containers for holy water can be sanctified on the altar; the larger the altar, the more containers can be placed on it to receive a blessing. Holy water that is place in another, more profane container must be used before sunset or sunrise, whichever occurs first. Holy water in a sanctified container lasts forever.
The uses for holy water can be pretty varied. I would go with two primary uses: Turn Undead as a "zero-level" cleric and heal 1 point of damage on a Good reaction roll, if used immediately after combat. (If the wound was caused by a demon or undead creature, I'd allow a full 1d6 of healing, or 2d6 on a roll of 12.) A temporary circle of protection or Bless spell might be other possibilities; I'd handle these using the Cleric Without Spells approach.