Now that you're satisfied with your ChangeLog entries and you've returned to the mail buffer, you want to write some explanation text in the message. I'll let you do that. You also want to insert the ChangeLog entries corresponding to your patch, since they are usually much more readable than the patch itself.
Inserting your ChangeLog entries in the mail buffer is as simple as
typing C-c C-p l (patcher-mail-insert-change-logs). This
command places them just above the patch, with a short information line
(per ChangeLog file) on top.