I have read IRS 453 (https://www.irs.gov/taxtopics/tc453)
It says to enter the bad debt on form 8949 and attach a detailed statement.
"Report a nonbusiness bad debt as a short-term capital loss on Form 8949, Sales and Other Dispositions of Capital Assets, Part 1, line 1. Enter the name of the debtor and "bad debt statement attached" in column (a). Enter your basis in the bad debt in column (e) and enter zero in column (d). Use a separate line for each bad debt. It's subject to the capital loss limitations. A nonbusiness bad debt deduction requires a separate detailed statement attached to your return"
The first part is easy enough. How do I attach a statement?