First I would connect the hose to the low pressure fitting before starting the car. This is close to the serpentine belt and it's easy to get hurt while the car is running. Have your can already screwed to the valve on your hose with the valve all the way counter clockwise.

Start the car and turn on your A/C full blast but not on recirculate

Screw the valve in, puncturing the can.

Unscrew the valve. The can should be getting cold and refridgerant flowing into your system. You shold start seeing the gausge increasing in pressure and fluctuating as the compressor cycles on and off.

My gauge is idiot proof, having red, yellow and green colors associated with the pressure values on it. These are nice indicators. If you are going to buy one -- look for this feature- it's easier to read the pressure range when the compresor is cycling.

If you want to get more technical, you can buy a dial thermometer and place it in the center vents on your dash to see how cool the air is getting. A nicely charged system should yield around 38degF vent temperature on full blast with an outside ambient temperature of 82degF.

If the can you are using is getting lighter and you notice a frost ring around the botom of of it, it's almost empty. Shake the can a bit and turn it upside down while charging to get the remainder out and to allow the oil in the can to get into the system. I usually shake the can as soon as I start charging the system just out of habit.

When you are satisfied with the temperature and the system has the proper pressure, turn the valve fully counterclockwise. This stops the flow in and out. Turn off the car and discunnect the hose from your low pressure fitting. replace the plastic cpa and you are good to go.

Good luck