Using the FaceBook app.
A status update
Start typing their name in the text box and a list of matching friends will show up. Select the one you want.
Your own pictures
Post the picture, then navigate to it and open it full screen. Next to the Like and Comment icons at the lower left will be a tag icon. Tap this, tap the person's face, then start typing thier name. As befor a list of matching friends will appear. If you are tagging someone who is not a Friend on FaceBook, type out the full name then tap Return on the keyboard.