Generate a patch according to emacs-mirror/CONTRIBUTE.

As described in Getting started, ensure melpa's whereabouts in init.el or .emacs:

(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/"))

Then

M-x package-refresh-contents RET
M-x package-initialize RET
M-x package-install RET magit-patch-changelog RET

And, finally add (before any mentions of magit) in your .emacs,

(with-eval-after-load 'magit
  (require 'magit-patch-changelog))

Alternatively, copy magit-patch-changelog.el to a directory among C-h v RET load-path and add (require 'magit-patch-changelog) to .emacs.

From the magit status buffer (C-x g):

W c e