To enter a description and an amount for Line 21 of Federal Form 1040 U.S. Individual Income Tax Return:
When you print your return, the text "See Attached" will print to the left of Line 8 on Schedule 1 (Form 1040) Additional Income and Adjustments to Income. A sheet attached to the return, titled "Other Income - Supporting Details for Schedule 1 (Form 1040), Line 21," will include both the description and amount entered.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.