How do I tag a picture or video?

1. Login to the desired Facebook profile.
2. Click on the Photos tab near the top of the profile.  
3. That will open a new page.  View a photo album by clicking on the album cover.
4. Select the image you want to tag by clicking on that photo's thumbnail.
5. That will open a new page.  Select the "Tag this Photo" link at the bottom, right side of the page displaying the photo.
6. Click on the person you want to tag.  Type a name for that area of the photo or choose one of your existing friend's names.

The same general principle works with videos.