I recommend writing into your contract that you will do the index because then your publisher can't decide to cheap out and not have one (look at how many books today don't) or tell you to "put it on your website." When I wrote my first book many moons ago, an older writer friend gave me a method for doing the index that is definitely low tech but works very well. First, you buy a whole lot of index cards. I would start with 1,000. When you receive the page proofs back from your publisher, you can do the index, but not before. Start on the first page. When you see something that you think ought to be indexed, take an index card and write "Smith, Joe, ax murderer, p. 1" and drop it in a box. Similarly for "Smith, Joe, p. 2" and so on. When the box fills up, get another box. When you reach the end of the manuscript, open a new document in your word processing software and pull a card from the box and input the information. Then grab another card and input that information in proper alphabetical order. When you reach the last card in the last box, your index is complete. It works. And it's cheap. Indexing software is expensive, several hundred dollars, and (I believe) only available for Windows computers.