When you commit, just use:
git commit -s
or
git commit --signoff
Or you can just write at the end of the commit message, on a line by itself separated by a blank line from the body of the commit:
Signed-off-by: Your Name <your.email@example.com>
If you already have the commit, use git commit -s --amend to add the above signoff line.
Or, if you are going to be sending this as a patch or patch series, you can use git format-patch -s or --signoff to add the signoff to the patch itself, without modifying the commit.