Engines need air to burn fuel, and turbos are just the compressors to give it to them. Turbochargers use hot exhaust gases to spin a compressor wheel, which shoves air into the intake tract and thus the combustion chambers. When the turbo pumps more air than the engine can ingest, that air compresses in the manifold to create pressure that we call boost. More boost equals more power, so to increase power safely, you must first know exactly how much boost your engine is making.
Locate one of the vacuum lines connected to your intake manifold. You can use one of the vacuum lines connected to the throttle body (main air valve, where the throttle cables connect), but make sure to use one of the lines that tap into the throttle body base on the engine-side of the throttle plate. Otherwise, you'll be getting a reading from the turbocharger tubing instead of the manifold pressure that matters to your engine.
Cut a 1-inch section out of the vacuum line you've selected. Slide a hose clamp over the ends of your tubing sections and slip the T-fitting into the hose ends and tighten the clamps with a flathead screwdriver.
Cut a 1-foot section of vacuum tube and slip a hose clamp over either end. Push one end of the vacuum tube over the third nipple on the T-fitting and tighten the hose clamp. To test turbo boost, insert a pressure gauge with a 1/8-inch NPT nipple into the other end of the hose and have an assistant rev the engine while you read the gauge. When not in use for testing, you can plug the hose with a bolt or pipe plug and tighten the hose clamp to seal it.
You might want to consider installing a permanent boost gauge inside your car to monitor pressure under real-world conditions. A number of companies sell universal boost gauges and mounting pods. To install the gauge, you need only drill a hole in your firewall and run a longer line from your T-fitting to the back of the boost gauge. You gauge kit will come with mounting instructions, brackets and the required hardware and plumbing. You can opt to connect the gauge to your dashboard lights for night-time monitoring, but it isn't required.