Photo tagging is pretty simple over all, but you may need some help with it. First, go to your photos and click on the photo you want to tag someone in.
When you click on the photo, it should look like this:
Click on the Add A Tag(+) under the picture. When you do this, you should be taken to a page that looks like this:
If you do not see the button to +Add Note/Tag Photo at the top, please try refreshing. If it is still not there, please try it in a different browser. Firefox would be a good one to try.
After you have clicked to +Add Note/Tag Photo, click anywhere on the picture and you will see a place to enter the user name you're looking for. Enter the person's name you'd like to tag and select it. Note how it lists my user number, that helps to narrow it down.
Should there be too many options and you're not sure which one is the person you're trying to tag, please talk to them and see if they'll change their name briefly to allow you to tag them. Most times when people want to be tagged, they are asking their friends, so they should be willing to help. Have them change their name to something completely random and crazy like X3FFtgif67babyone, something that is unique and should pull very few names up. As soon as you tag them, they can change their name back to normal.
Happy Tagging.