1. Fill the form on the link https://portal3.passportindia.gov.in/Online/index.html
2. Along the form, please take copy of renunciation certificate that you receive from the embassy and Indian original passport
3. You pay around 26 Euro for Adult and for each kid around 83 euro
4. the process will take 2 months from the Indian Embassy, they will reach out the applicant and invite to collect the Cancelled Indian Passport & Approved Paper on the Cancellation.
5. Book appointment for OCI so that you will be on time of getting OCI
https://www.indianembassybrussels.gov.in/schedule-appointment.php