Automotive AC diagnostics procedures vary with your problems. Start by connecting the manifold gauge set to the respective high and low pressure fittings again you can find the location of service fittings in your service manual.
Close all the doors and windows and set up a fan in front of the radiator to simulate going down the road. Also to simulate road conditions it is recommended to raise the idle to about 1500 to 2000 RPMs. Now you can take accurate pressure readings on the high and low side and compare them to specifications.
You can also take an AC temperature output reading at the center duct with the blower on a medium speed or lower. With the pressure readings and the output temperature recorded this will give you a good baseline to start your diagnostics. Tree charts in your auto repair manual will guide you through the specific repair of problems indicated by your readings.