Voluntary acquisition of foreign citizenship automatically cancels your Indian Citizenship as per the constitution of India. The government of India says you voluntarily acquire citizenship when you either (a) register as citizen, (b) go through naturalization or (c) apply for a passport. This fact remains unchanged since the consititution was written, even today, after OCI was implemented.
The correct process to follow is to send you Indian passport in ASAP after naturalization/registration for cancellation. This is a condition on the passport, and a part of gazetted rules of Indian Citizenship.
If for some reason, you have been unable to get your passport cancelled, it is not a major issue - the main concern is that it has not been used. Good practice though is to cancel it ASAP.
In the context of OCI, a good time to have your passport cancelled (if you have not already done so) is during the application process. You will need to pay for return postage if you are mailing your application [either send a self addressed and stamped priority mail/express mail envelope or a bankers check about $15 - please check with consulate] so they can return the cancelled passport to you. There is no charge or form to have a passport cancelled (only postage). Write a letter to the effect "Dear Sir/Madam, I acquired XYZ citizenship on ddddd, and in accordance with the conditions of Indian passports, I wish to have my Indian Passport (enclosed) officially cancelled. Please return the cancelled passport to [address]. Enclosed is a return postage paid envelope/return postage of $xx"
Some consulates want a copy of your new passport and/or naturalisation certificate (and the court order if you changed your name). It is worth sending these along just in case.