Why not use the indexing feature of a word processor?
Literary agents, editors and book project managers will advise against preparing your own index. A professional indexer will provide an objectivity that an author will not have. This objectivity is needed to produce an excellent index. Also, after reading and rereading, an author or editor will definitely miss the last few typographical errors and other small mistakes. I will find these and pass them along gratis.
Automatic indexing existed long before there were computers, and is based on listing all words in the text, throwing out noise words such as a, an, the, in and the like. A famous index of this sort is the concordance to the Bible. This kind of index is highly unsatisfactory for most use and does not take into account synonyms or the intellectual task of grouping concepts.
The more recent indexing tools on word processors are awkward to use, and they take time away from writing, editing, and academic study.