How to connect a vacuum gauge?

There's all kinds of vacuum hoses and ports on all engines. There will be an emissions sticker under the hood that shows the vacuum hose routing. If you can find a port that wasn't needed for the list of optional equipment on your car, there will be a rubber cap on it. You can just pull that cap off, then connect the hose for the vacuum gauge to that. You can also cut a vacuum hose, plug a three-way tee into it to provide another port, and hook the gauge to that.

You're after a port or hose that is attached directly to intake manifold vacuum. There are going to be a lot of other hoses that look the same but are for other purposes. Some have vacuum switched on and off depending on their purpose and the current conditions. Some use small vacuum signals to control other systems. The best is to look right on the intake manifold for a capped port or a hose you can cut.