This feature is of no interest to me at this time, so for practical how-to advice I’ll refer you to many of the other, excellent photography resources such as Victoria Bampton, the Lightroom Queen.
That said, I did want to provide some advance warning to anyone with a large catalog. (Note that the normal process of the catalog upgrade to make it LR6 compatible should be about what you’ve experienced in the past.)
If you elect to have Lightroom index for faces, then the next step could take a very long time indeed. One tester with a catalog of hundreds of thousands of images reported that it took over 150 hours for Lightroom to finish it’s face indexing. You have been warned.
Are you excited about face tagging? How will you use it?