A letter of authorization is handed to a bank to allow a second party to manage either your account in general (a general letter of authorization) or a specific action (such as cashing in checks on your behalf).
Here is a standard template I have quickly drafted for you, I hope this helps:
(any other contact details)
I am writing to authorize (name) to act on my
behalf in all matters relating to (either a specific purpose, or mention that it is general access they will be granted), including verification and signing of any related documents.
This person's signature shall have all the weight and authority of my own.
This authorization is valid until (date OR until further notice)
(Specimen Signature and name)
(What ID will be submitted)
Your Signature and name in print.