Adding to the article above ...

Editing a well-prepared index can be a pleasure,

the estimable Chicago Manual of Style tells us,

A poorly prepared one, however, presents serious problems. As an editor, you cannot remake a really bad index. If an index cannot be repaired, you have two choices: omit it or have a new one made ...

Also:

As a very rough guide, an average of five references per text page in a book will yield a modest index (one-fiftieth the length of the text), whereas fifteen or more will yield a fairly long index (one-twentieth the length of the text). [Printed books incur a marginal cost per word, remember.]

The old-fashioned way. Indexers used to handwrite or type preliminary entries and subentries on 3 x 5-inch index cards, then alphabetize and edit the cards, and finally type the index, while further refining it, on 8 1/2 x 11-inch sheets.

And finally,

For greatly expanded coverage of the present guidelines, along with alternative usages, consult Nancy Mulvany’s Indexing Books .... Anyone likely to prepare a number of indexes should acquire that work.

https://press.uchicago.edu/Misc/Chicago/CHIIndexingComplete....

Wasn’t this on the homepage yesterday?