Advise on transfering flat sharefrom joint ownership

Can you please advise me in the following matter? My ex husband and I have joint ownership of a 2BHK flat in Navi Mumbai, which we purchased when we were married. We have been divorced for few years now. He does not want his share in the flat anymore as he is moving to Canada. We are both British citizens but I am OCI holder. He does not have OCI. He wants to transfer his property to me which will give me single ownership. I want to sell the flat in the future. What would be the best and correct way to get single ownership? I have read about transfer of shares, gift deed, relinquishing rights and power of attorney. Does the transfer need to be registered and tax paid to government for the share transfer? I want to pay the minimum amount of money towards the flat as I want to sell it in future. Please advise me on the pros and cons of each if possible. He will not be able to come for any sale in future. I live in the UK and not possible to come to India regularly. Thank you in advance.Looking forward to your reply.