Please create a new ticket to track the issue unless your patch is a patch for an issue that is already ticketed.

Then run the following darcs commands to generate a darcs patch file named WHAT.diff:

  darcs record local/path/to/file(s)
  darcs send -o WHAT.diff

And attach it to the ticket.

Please use a filename like *.diff (instead of *.patch) so that Trac will let other developers view it without first requiring them to download it first. (Trac's content-type logic maps *.patch to some application/blahblah MIME type, which it refuses to display as HTML)

If you don't have darcs or don't want to use it, just attach a normal old diff.