The following points will help with learning to use imagery effectively:
As with most sports psychology techniques, it is often best to start gently so that the basic skills can be fully learned in a low stress environment. This means that you can be more confident of the effectiveness of these skills when you need to put them to the test.
Initially start using only 5 minutes of imagery a day, perhaps when you have just got into bed, or when you wake up in the morning. The number of minutes can be expanded as time goes on: typically many champions will do 15 minutes/day, although this may go as high as 1 hour/day just before a major competition.
Similarly, start using imagery in a quiet, relaxed environment in which there are few distractions. Slowly experiment with using it in increasingly disturbed situations until you are comfortable with using imagery in the most distracting environments such as high level events.
It is important too to use imagery systematically: get into the habit of practising techniques in your mind before executing the in practice, and of using stress management imagery routinely. A habitual routine use of imagery will bring its benefits almost automatically when you are under stress.