With the port cleaned, you're ready to attach the rubber hose that links the car to the pressure gauge. they don't screw on, but instead use a quick and simple latching mechanism. Too attach the hose to the low pressure port, pull the outside of the fitting back, slide it over the low pressure port, then release it.
Now start the engine and turn the air conditioning on high. Take a look at the gauge and you'll see how much pressure your system is building. Give it a few minutes to get the pressure up and equalize, then you can take an accurate reading to see how badly (or not) you need to recharge the AC system.