Both of you should agree what words you'll use to tell her when to stop and start stimulation. Words are necessary because nonverbal signals are easily misunderstood. Do whatever is necessary for both of you to feel relaxed and comfortable before beginning an exercise.
Some couples like to start right in with penile stimulation.
Others prefer to begin with holding, or massage. And in still others, the man sexually stimulates his partner before she returns the favor. There is no rule to fit all couples. Do what is right for the two of you. It's essential that your partner take her comfort into consideration in deciding what positions to employ.