Did you go to a therapist, or do it with a tape or something?
I did it with a tape and a book. Basically, you need to write a "script" that includes both your goal and the steps needed to achieve it. You associate certain visual images with certain words and tie all that to a physically and mentally relaxed state.
You have to rehearse your script a lot. Two or three times a day for a couple weeks and then at least once a day after that. Eventually you will be able to relax just by mentally saying a word or two.
Try to find a book about self-hypnosis in general, that way you can tailor the script to meet your own needs. You know yourself better than any therapist can.