Not that I would recomend that you try to do this without someone that is experienced, but.
1) get a pair of safety glasses and wear them before you try using a gauge set, if you screw it up you will want to protect your eyes.
2) make sure both of the valves on the gauge set are closed (the red and the blue),before you hook the gauge set up.
3) connect and disconnect the gauges when the engine is off!
4) Add freon only to the low pressure side (that's the blue valve)
5) make sure the freon can is right side up!!! Upside down will put liquid freon into the low pressure side, VERY BAD!!
6) The freon can will freeze as you charge your system, to prevent the can from freezing, put it into a bucket of warm water. Do not use any other type of heat,,IE open flame on the can to keep it warm.
Buy a book, and read up before you start, there is too much to cover in this type of forum. and make sure to wear safety glasses!