Give the book a thorough read underlining or otherwise highlighting those word you can want to include in the your index you will be surprised how quickly it will go and you will be in control of the content as you should be.
I recommend rather that you use the free software Open Office Writer to prepare the index. Once you have got the hang of it, it is easy to update the index automatically. It takes the tedium out of the job.
Only index points that your reader could conceivably find useful. Remember to use lowercase consistently for each entry unless it is a proper noun.
As Dennis says, your index terms are like keywords that someone might search for. As an average, you might have 2-4 terms per page that go into your index.
Apache Open-Office Writer is about as friendly and free as they come.
In addition, you may find https://labs.translated.net/terminology-extraction/ of use in suggesting which terms to index. But remember, the reader wants to have concepts indexed, and that has to be done by hand.