Using the git tag command,

git tag -a v1.2.0 6550194 -m "Add new functions, classes, and reoganized"

where:

• -a signifies that you are appending to the old commit
• 6550194 is the hash for the commit
• -m '<insert comments>' signifies your comment for adding the tag

the final step is to push the tag to the remote repository using

git push --tags

then you are done. Next step is to create a release using that tag — see GitHub Releases & Tags.