You can try talking to the person first, face to face calmly to resolve the conflict. If they still are causing issues, or are abusing you in any way, please consult your appointed lawyer and explain the circumstances to the lawyer. If the lawyer sees fit, they will decide to send the legal notice.
To write a legal notice, you will need a lawyer who is good at drafting such notices. Explain the whole situation, mention the names of every person that was present in the situation, and if there is a recording, let the lawyer listen to the recording. A legal notice is considered as a final warning before filing a suit against the person (defendant). If your lawyer does decide to send the legal notice, make sure it reaches the person and ONLY that person, don't send it through a friend or anyone that's not related to the case. English is the preferred language, but you can write a legal notice in any Indian language. The lawyer usually will keep one copy of the legal notice with themselves as well. The person will be given a time period to reply to the notice, and to settle the conflict among yourselves. If they don't reply, please notify your lawyer.
Hope this helped,