Player's Handbook II (3.5) / Divine Conversion / p193
This is a sidebar at the bottom of the referenced page, which is part of a section on rules for retraining. If you don't have the book, here is the direct quote:
DIVINE CONVERSION
As noted in the Player’s Handbook, a cleric who grossly violates the
code of conduct imposed by his deity loses all spells and class
features and cannot attain any more levels as a cleric of that deity.
All these penalties remain in effect until he atones. But what if he
doesn’t want to atone? What if a cleric of Hextor finds new meaning
and purpose in serving Heironeous after a dramatic conversion
experience? Such a character need not become a multiclass ex-cleric of
Hextor/cleric of Heironeous. Instead, Heironeous can simply reinstate
the character’s cleric powers once he has proven his loyalty, talent,
and ability.
A cleric who changes his patron deity must complete a quest to prove
his devotion to his new patron. The nature of the quest depends on the
deity, and it always clearly reflects the deity’s alignment as well as
his or her goals and beliefs. To start the process, the cleric must
voluntarily accept a geas/quest spell cast by a higher-level cleric
of his new deity. During the quest, the cleric has no access to spells
or cleric class features—except his weapon and armor proficiencies,
which he does not forfeit.
Upon completing the quest, the cleric receives the benefit of an
atonement spell from a cleric of the new deity. The character then becomes a cleric of the new deity and is inducted into the clergy
during an appropriate ceremony of the DM’s choosing. After selecting
two of the new deity’s domains in lieu of his old ones, the character
has all the powers and abilities of his previous cleric level, plus
the granted powers of his new domains.
This method is the only one by which a cleric can change his deity.
The retraining rules can’t be used to accomplish this task—it is
simply too substantial a change in the character’s identity (not to
mention his source of power) to chalk up to a bit of practice in his
off hours.
This answer essentially reflects the ruling already found in the Forgotten Realms Campaign Setting, but it may be useful to know it's also located in a setting-independent rulebook, just in case there are any sticklers out there.
Casting a spell takes a magic-user's entire round. They may not take any other actions, including moving. The number of segments listed for a spell is only used to figure out when during a round the spell completes, which can be important for figuring out whether the spell is interrupted (and spoiled).
For a complete breakdown of the spellcasting process during combat, see page 65 of the DMG.
Learning AD&D for the first time from the original books takes a ridiculous amount of dedication due to the bizarre organisation, the rambling discoursive style, and the way a single rule can be spread across multiple chapters or even across the three core books. I certainly didn't learn the fundaments that way – like most AD&D players, I learned by playing with people who already knew how to play.
To learn directly from the books requires dedicating a lot of time, and using that time to read and re-read them, from cover to cover and in bits and pieces, while putting them to use in an ongoing campaign. A large tolerance for doing it "wrong" for a good while is helpful.
As an alternative, I can recommend reading OSRIC, which is a retro clone of AD&D. Its advantages include being more concise (it's only about 40 pages of rules plus 80 pages of spells), organised more like how we expect rules to be organised today, and free. It makes a good companion to the original books, and can help clarify rules confusion since it's much easier to find everything.
The conciseness of OSRIC is also its downside: it lacks the rambling commentary that puts the rules in context, and which sometimes provide more nuanced guidance that falls somewhere between "background" and "rules". For example, OSRIC makes no mention of spellcasting preventing crouching when a spell has somatic components. I never knew about that rule(?) until I re-read the several different sections in the PHB and DMG about spellcasting while I was looking for a citation for this answer.
Best Answer
The Player's Handbook lists holy water as available for purchase for 25 gp on page 36 under Religious Items as Water, Holy, vial.
The Dungeon Master's Guide contains instructions for clerics on how to manufacture holy (and unholy) water on pages 114-115 in the section entitled Creation of Holy/Unholy Water. Since the instructions run about 500 words and are in the DMG, it's probably a good idea to ask the DM directly how it's done in that DM's campaign.