Usually when you buy a vacuum guage it comes with instructions or a chart
that will tell you what the different readings mean. Mine has it written on
the face of the guage. Also I think the Haynes manuals detail how to use a
vacuum guage, and I'm sure there are plenty of "how to" articles on the
'net.

If you find there is a leak somewhere, check all your vacuum hoses for
cracks, loose connections, breaks, kinked, collapses, etc. Check the intake
pipe and air filter box for holes/blockage/loose joints/etc. If all checks
out, maybe it's a gasket somewhere. Take a can of throttle body cleaner,
other people probably use other things, and spray it around the mating
surfaces of the intake manifolds. If there is an intake vacuum leak the
cleaner will be sucked into the hole and the engine speed will change. I
think I've heard people use propane as well.

Your timing could also be out, but a vacuum guage should be helpful in
determining if it's a vacuum leak or a timing issue.

Steve