There are no official rules for this, and as far as I can tell, no-one has suggested house rules for it anywhere online either.
That passage you quote is all the official answer the game provides, and it's (almost certainly) vague on purpose—it's RPG-book code for "we are leaving this up to the GM":
Rumors exist […] those with any actual knowledge of these procedures are dead, hidden, or not talking.
It is common in RPGs to leave things undeveloped when they're all of very niche, very specific, and rare. This used to be common in RPGs for even less niche/specific/rare things, because designers both recognised that every GM would alter the world anyway and leaving blanks made that easier, and because designers and developers have only so much time. Besides, when games defined everything, the publisher is inevitably lambasted for not allowing any room for GMs to customise the game and world for their home campaigns.
So, it's a long-established custom for even solidly-defined games to leave some of these lacunae for GMs to play with, if they so desire—especially in an example like this which would be open to player abuse. (Just imagine if there were official rules for these forgotten, unknown, or possible non-existent rumoured procedures: suddenly, somehow, every PC mage buying their first ioun stone would somehow know these unknowable things.) You still see it as a matter of course in official settings, where the GM can't avoid providing their own details to be able to run a campaign in the setting. This is still happens in rule sets as well, since not every possibility can be accounted for by the designers—and though some games do try to minimise these unspecified parts of the rules, eliminating every gap is impossible.
But when mention it at all, then? Because by mentioning it, the possible existence of such procedures is now official. The designers are backing up any GM who wants to flesh this detail out; a player that protests "that's not in the rules!" is (besides missing the point of the GM's role as worldbuilder even in existing settings) going to have that paragraph pointed out as a rebuttal.
This custom is easily confused with "Rule Zero", but isn't the same. Rule 0 gives GMs permission to change the rules (or alternatively, it acknowledges that the rules need the GM's consent to exist at their table). In contrast, gaps like these require GMs to fill them with their own creations in order for them to see play, whether that's creating NPCs and villages on a blank space on the map, or creating long-forgotten ioun stone rituals for players to discover as the reward of some quest.