See IRS Form 56.  From the Instructions:

Who Should File

Form 56 should be filed by a fiduciary (see Definitions below) to notify the IRS of the creation or termination of a fiduciary relationship under section 6903. For example, if you are acting as fiduciary for an individual, a decedent’s estate, or a trust, you may file Form 56.

A fiduciary is treated by the IRS as if he or she is actually the taxpayer. Upon appointment, the fiduciary automatically has both the right and the responsibility to undertake all actions the taxpayer is required to perform. For example, the fiduciary must file returns and pay any taxes due on behalf of the taxpayer.

A fiduciary is any person in a position of confidence acting on behalf of any other person. A fiduciary assumes the powers, rights, duties, and privileges of the person or entity on whose behalf he or she is acting. Examples of fiduciaries include administrators, conservators, designees, executors, guardians, receivers, trustees of a trust, trustees in bankruptcy, personal representatives, persons in possession of property of a decedent’s estate, or debtors-in-possession of assets in any bankruptcy proceeding by order of the court.