where and how to measure?

trying to figure out the measurements of my backspacing and off set i will need for my astro. how will i go about figuring this out? where to measure? i'm having a hard time finding info on this! thank you

http://www.streetsourcemag.com/forum/topic.aspx?topic_id=57855&Topic_title=Offset+...+Im+lost....+measuring&forum_id=18&forum_title=Wheel+Questions

thank you.