Start with the old Tracking URL. If you don't have one and only have a Report ID, please see FAQ Entry: Getting a Tracking URL from a Report ID.
Browse to that old Tracking URL.
Click the "View entire message" Link.
Copy everything except the first two lines (which look like the following).
CODE
CLICK 'BACK' BUTTON TO RETURN TO SPAMCOP
################################################################################
Alternatively, you can delete those two lines after you paste.################################################################################
Go back in your Browser, or to a new Parser window where you can paste your spam.
Paste the spam into the "Paste entire spam" Textbox.
Click on the "Process Spam" Button. This displays a current parse of the reported message, which includes the Tracking URL.
Please make sure that you don't send more than one report to the same email address for the same spam.
Also, please make sure that any Tracking URL you post either has already been reported or has had its reporting cancelled, to avoid other people reporting your spam or tests.
Edit: Jeff G. added the Report ID reference and the last paragraph.