First you must turn on "Full Headers".
From your Yahoo! mail account, click on "Mail Preference".
Scroll down the page to "Message Headers"
Click on the "all" radio button.
Save your preferences at the bottom of the page.
View the message you want to report.
If the message is in plain text, copy/paste to the form.
If the message to be reported is HTML:
View the message and copy the complete headers.
Paste these in the the form window, then add a blank line.
Go back to the Yahoo! window and select to "Forward" the message as "inline text" (drop down menu).
Scroll down the message to the start of the message body. (The first line of the HTML body will usually begin <HTML).
Copy the body of the message and paste into the the form parsing window.
Make sure a blank line remains between the header and body.