Use the directions and read the safety warnings that came with your vacuum gauge.

When testing, connect vacuum gauge to a vacuum port on intake manifold. You want the reading to come after the throttle plate. If you do not have to drive the vehicle, the brake booster vacuum hose is a good place to tap in. As you will get good volume. It is important not to drive the vehicle with a gauge tied into brake booster vacuum hose.

Engine Vacuum Specifications

Engine Vacuum – How to Test
Snap throttle Jumps to 2″ then on decel moves to 25″ Healthy
Snap throttle Jumps to 1″ then on decel moves to 22″ Fair but worn
Idle 8″ or less Vacuum leak, loose intake manifold
Idle Fluctuated between 14″ – 19″ Worn valve guides or head gasket blown between 2 cylinders
Idle Reading drops from normal Burnt valve, valve stuck open, misfiring spark plug
Idle 8″ – 14″ Valve timing incorrect or large camshaft overlap
Idle 14″ – 16″ Ignition timing wrong
Idle Moves between 12″ – 16″ Idle mixture incorrect
Slow engine speed rise Needle falls then rises suddenly Blocked exhaustWeak valve springs
3000 rpm Needle fluctuates and worsens with higher rpm Weak valve springs