My advice would be to send the Form 8949 or Schedule D and include the bad debt statement with that form if you already used e-file to send your tax return. It must be accepted before you mail, but it should be mailed within 3 days of acceptance by the IRS.
If you are e-filing your tax return, then mail your statements (Schedule D, Form 8949 if applicable, bad debt documents) along with Form 8453 to:
Internal Revenue Service
Attn: Shipping and Receiving, 0254
Receipt and Control Branch
Austin, TX 73344-0254
If you need a blank Form 8453, you can download this pdf, enter your address information and check the box for Form 8949 (this form is really just a cover sheet)
If you did not e-file then you should print your return and mail it along with the bad debt document(s).
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
