Use a good 4 foot level. Start where you know you need the depth and work around the room. I did this in my basement and was off only about 1/4".
Armstrong had a good program you could download which made estimating materials and centering very easy. They also have detailed instructions.http://www.armstrong.com/armstrong_home.jsp
Its important to get the end pieces the correct size so the ceiling is balanced. This is hard with irregularly shaped layouts. The Armstrong program can help.
Note that the material list breaks it down by kits. You don't need to buy kits. For the mains, use the 12-foot sections available at home centers.
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