2002 Dodge Dakota • 100,000 miles

How do I check my ac freon level I have the gauge do I have to turn truck on and ac then put gauge on to check or do I have to do it when truck is off? I did check it with everything off and it was past blue and into yellow almost to red if it is to full how do I let some out and is this why it is cold but not real cold?
June 29, 2012.

Hook the gauge up and start the truck let it idle for a few minutes and then turn on AC and set the AC to Max-read the gauge at this time-

Jun 29, 2012.