• Back-of-the-book indexes -- those appended at the back of printed books
  
• Journal indexes -- where an index, often in a standalone volume, covers several issues or volumes of serial journals
  
• Web indexes -- where an index covers a website, usually with active links to pages or locations
  

For printed back-of-the-book indexes, indexing begins when the publisher has typeset everything except the index.  A set of galley proofs is sent to the indexer, usually at the same time another set is sent to the author for proofreading.

When the indexer receives the proofs, a brief period of intense work will ensue.  During this time the indexer reads the book with deep concern, crystalizing each pertinent idea into a succinct phrase that would lead any reader to that particular passage in the text.

This is in no way a mechanical process, but an unpredictable and creative process whereby the indexer strives to identify the book's discrete ideas and to formulate concise keys that lead to them.  Authors sometimes express surprise at the substantive depth and breadth of the indexer's reading.

The index is compiled using special computer software that sorts the headings and formats the index according to the author's wishes and the publisher's specifications.  I use a program called Sky Index.  The author is sometimes consulted for advice on what terms to include or exclude; the publisher has "house specs" that dictate the physical appearance of the index.

So essential did I consider an index to be to every book, that I proposed to bring a bill into Parliament to deprive an author who published a book without an index of the privilege of copyright; and moreover, to subject him, for his offense, to a pecuniary penalty. —John Campbell, Lives of the Chief Justices of England

During the indexing period, it can be important that indexers, editors, and authors are able to contact one another. The process of indexing sometimes reveals errors that are not easily caught in front-to-back proofreading--ordinarily the author will want to verify that these are genuine errors and mark corrections on the author proofs.

After the initial compilation of headings, there is a lengthy editing stage.  Duplicate or similar entries are combined.  Cross-references are added to link related headings.  Succinct phrases are made even more succinct.  In general, editing allows the structure of the index to fully come to fruition so that a reader can easily navigate its space.

When the index is finished, it is sent to the publisher and sometimes also to the author for proofreading.  Minor changes may be made before it is typeset and printed.  Generally at this point the indexer's work is done. An invoice for payment will be sent to whoever made the initial contract.