Indexing a book is like doing plumbing. Everyone knows what an index (tap/toilet/other appliance of choice) looks like. We mostly know what they do (I’m never terribly sure if a bidet in a hotel room is working properly). Some of us know a good or a bad installation when we see it. Some of us put up with bad plumbing because we can’t fix the one we’ve got (leaky tap, grumbling radiators etc). Yes, you can do plumbing by grabbing a book, watching a YouTube video or just getting in there. But it might take you a long time, you might not have all the parts or tools you need for the job, and the result probably won’t be as good as a professional job.
So, as with plumbers, if you need an indexer, get someone who has been on a recognised course, done the training with expert tutors, and got the professional equipment. When your time is money, it makes sense to find a professional.