How do you file an amended tax return for a grantor letter just received?

You would enter the information reported on the Grantor Tax Advice Letter as if it were reported to you on a Form 1041 K-1.

After you have entered the initial information about the trust, you will get to the section where you can select the boxes to fill in.  Put a check beside Boxes 1-5.  This will allow you a place to enter ordinary and rental income, plus long-term and short-term capital gains (box 3 and box 4a).  The information will be transferred to the appropriate section of your Form 1040.  

A lot of grantor letters include expenses which can't be deducted and are purely informational, like administrative and legal fees.

Use the following steps to get started:

• Open your return and search for "k-1."
• The search results will give you an option to “Jump to K-1”
• Click on the blue “Jump to K-1” link
• Answer Yes to get started
• Choose Start beside Estates or Trusts (Form 1041) and enter your information.