There are now two ways to approach this:
- Using gpg and generating keys
- Using Kryptonite by krypt.co
This Gist explains how to do this using gpg in a step-by-step fashion. Kryptonite is actually wickedly easy to use-but you will still need to follow the instructions
For using a GUI-based GIT tool such as Tower or Github Desktop, follow the steps here for signing with either GPG or Krypt.co.