How do you learn how do diagram a sentence ?

Answer 1

This website is the best I've seen for explaining how to diagram sentences:

http://members.cox.net/teachro/

Click on Diagramming to get started. You can use the Grammar section if you need explanations of any parts of speech or parts of sentences.

Answer 2

Slowly and with lots of practice.

These are excellent sites that explain diagramming.

http://drb.lifestreamcenter.net/Lessons/TS/diagram.htm

http://grammar.ccc.commnet.edu/grammar/diagrams2/diagrams_frames.htm