I have deleted the website. It was on wordpress. Now I want to let Google know that the website is permanently deleted while also informing users that this site is no longer available. The users will obviously continue to come on different links. So I want them to be redirected to a page index.html in the root directory to inform them about unavailability of website.
How do I do that. So that search engines quickly understand and remove the site from results. While people coming from other non-organic links are also redirected properly to a page.
Hm. Before reading the post I saw the title and thought "That's really two questions: What to do and then how to do it."
So the site is really going bye-bye, not getting redirected to a new location? But you're letting the domain die a natural death, not pulling the DNS plug right away?
If it were me, I would return a blanket 410 to all requests for the site. 410 (Gone) is factually correct, so why not use it. Combine it with a very nice, thoughtfully designed custom 410 page that tells people anything you want to tell them. Like a, well, not like a store going out of business. Think of it more like a professional person retiring. "Thank you for your support over the years, it's been real, here are some other sites you might like."
Do not cut & paste: I'm making this up as I go along. The 410-- here expressed with the [G] flag-- tells both humans and search engines that everything is gone. Humans will get automatically routed to the 410 page; search engines will just note the 410 and gradually slow down their crawling.