Question

how do diagram a sententce

Answer 1

http://members.cox.net/teachro/
This is the <b>very best</b> site online by which to learn diagramming.

Click on the <b>Diagramming</b> link to find all the different types of sentences and elements in sentences, with very clear directions and graphics to show you how.

If you have questions about any grammatical elements that you have to diagram, then click on the <b>Grammar Practice</b> link.

Answer 2

Here are some other websites, but I still think the one I gave you above is far and away the best of them all!

http://grammar.ccc.commnet.edu/grammar/diagrams/diagrams.htm

and these:
http://www.google.com/search?q=how+to+diagram+sentence&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GGIE_en