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:

  darcs record local/path/to/file(s)
  darcs send -o patch.darcspatch

And attach it to the ticket.

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