At the top of every patch you should include a description of the change you are making as well as your motivation.
The sign-off is a simple line at the end of the patch description, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. The "Developer's Certificate of Origin" pledge is taken from the Linux Kernel and the rules are pretty simple:
Developer's Certificate of Origin 1.1
By making a contribution to this project, I certify that:
(a) The contribution was created in whole or in part by me and I
have the right to submit it under the open source license
indicated in the file; or
(b) The contribution is based upon previous work that, to the best
of my knowledge, is covered under an appropriate open source
license and I have the right under that license to submit that
work with modifications, whether created in whole or in part
by me, under the same open source license (unless I am
permitted to submit under a different license), as indicated
in the file; or
(c) The contribution was provided directly to me by some other
person who certified (a), (b) or (c) and I have not modified
it.
(d) I understand and agree that this project and the contribution
are public and that a record of the contribution (including all
personal information I submit with it, including my sign-off) is
maintained indefinitely and may be redistributed consistent with
this project or the open source license(s) involved.
... then you just add a line to the bottom of your patch description, with your real name, saying:
Signed-off-by: Saalim Quadri <[email protected]>
You can add this to your commit description in git with git commit -s
See this guide if you've never done this before.