Last updated September 21, 2022
This Gist explains how to do this using gpg in a step-by-step fashion. Previously, krypt.co was heavily mentioned, but I've only recently learned they were acquired by Akamai and no longer update their previous free products. Those mentions have been removed.
For using a GUI-based GIT tool such as Tower or Github Desktop, follow the steps here for signing your commits with GPG.
There has been a number of comments on this gist regarding some issues around the pinentry-program and M1 Macs. I've finally gotten a chance to try things out on an M1 and I've updated the documentation in 2-using-gpg.md to reflect my findings.