I usually start out by trying to identify what the feeling is. I usually use a feelings chart, such as:

Or use a smiley chart. Then I try to back track to figure out why I'm feeling that way. It's usually either something that has happened or that I fear will happen. I find journalling helps. I usually start off with "I'm feeling _______ because of _______". It gets easier as you practice. There are also apps for smart phones that let you chart your moods. These can be helpful, too.