How do I buy MBS for someone?

Could you please elaborate on the steps required to purchase MBS, or mortgage-backed securities, on behalf of someone else? I understand these investments can be complex, so I'm curious about the process from start to finish. Do I need to have a broker or financial advisor to assist me? What paperwork and documentation would be required to ensure the transaction is legitimate and complies with all relevant regulations? Lastly, how do I ensure that the person I'm buying the MBS for is fully informed and consents to the investment?