If you are planning to write a book or have one already written, then you need an ISBN to it. There are two case scenarios- if you want your publisher to do it, he can get it done at the same price, but if you're going to self-publish it, then you would want to follow the steps given below:
Before applying for ISBN, you must finalize the font style, size of the text, margins, and everything. DON'T forget to mention the number of pages; do it correctly because they won't allow you to alter the number of pages later!
Get your book printed just the way you want it to be because you're going to need the front and the back cover while sending the application.
Finalize the maximum retail price you want to sell it. The typical cost a book consists of is printing, binding, storage, marketing, publisher's commission, etc., so don't forget to make some profit!
Write down the application enclosed with a copy of the cover page, photocopy of ID proof, and a self-addressed envelope for return communication.
Send your draft application enclosed with the documents mentioned above on the given address-
Raja Rammohun Roy National Agency for ISBN
West Block-I, Wing-6, 2nd Floor,
Sector -I, R.K. Puram,