Select a photo with people and open it in Single Photo View.
Click the Face Tagging icon in the right sidebar.
If a face is detected:
If the person is not previously tagged, enter their name and create a new tag.
If the person is previously tagged, select their name from the list.
If multiple faces are detected, repeat step 3 for each person.
If you don't know or don't want to tag a face, select Ignore.