I write mostly non-fiction, which doesn’t often utilize the element of an index. If you write non-fiction, however, or intend to some day, this post might help.
What is an index?
Non-fiction books often require the author to establish one, and old-school publishing may ask you to include an index or list of words to be included. This can also cost you money.
If you self-publish, you want to create the index so your reader and cross reference information.
This video shows a basic tutorial on how to create and edit an index in MS Word;
I recommend you browse their channel playlists for more tutorials that are helpful for writers.
But don’t worry, Openoffice (free word processor and office software suite) can also can create indexes much the same way. Here is a PDF document which shows you how to add index, but also table of contents.