Take a deep breath. I feel your pain. I'm a historical re-enactor, so none of the patterns are correct anyway. If this is your first project, go VERY slow. You will probably have to redo it. If you are making clothes, I highly recommend you buy cheep muslin fabric and make a test run first. Muslin goes for about a dollar a yard - cheaper if you use a coupon, so it's not a big investment if it doesn't work. If the pattern is skipping instructions (and they do) try to think about it like you would a puzzle and do the logical thing.
http://www.mccallpattern.com/charts/index.htm
Here is a measurement chart for McCall's patterns.
And remember: The instructions are more guidelines than actual rules. :)