Authorization letter?

you merely notify the tenants in writing, dated etc, that your brother, whom you identify, is authorized to act as your agent

You must start by getting permission from your landlord first. Most do not allow
subletting.

Are you the owner so he be managing it? If so, just draft a letter to the tenants tell them his name and contact info letting them know that he will be managing the apartments for you and collecting rents. Since there is a lot of scams going on, you also may want to take him around and introduce him to your tenants personally if possible.

first you need to review your state laws, many states require a third party to collect rents and act as a PMC to have a active real estate license, while even if your state requires such you probably could get away with it but if something comes up and you need to go to court it will become an issue

Nothing formall is necessary. Just a note to the tenants that your brother is your property manager.

As you own the property, you will have to contact your government and apply to be a Landlord. You will need to state that someone will be renting a property on your behalf also.

Google it