In Tantra, the very first step is to surrender yourself to the therapist. But Subject is how do you surrender. And what do u understand with surrender. Surrender means you to stay calm. First close your eyelids. Let each cells and tissues of your body be relaxed. Be relaxed to recognize the lightest touch on your skin. In fact, any part of skin in your body. Stay as Calm as even a lightest blow of air on skin be noticed. And this only can be possible when you concentrate on your skin, when you meditate on your own body.
When you surrender, you need to be calm and to be calm your mind must be in rest. Its okay if mind does not stay in rest but doing regular practice makes it easy to control it. That's why the regular yoga practitioners can surrender very easily. What you need to do is only to think about your body. Just think how your body is separating energy from the cells and amassing over the outer wall of body skin. This will make you feel the energetic waves all into your body.
When the first touch given to you by the therapist, you would feel the waves similar to like putting finger into a calm water filled in bowl. That waves drags you to get disturbed in your mind-calmness. It is because a different but more energetic waves from different (therapist's) body is being interspersed into the waves of your body. At first interaction of energetic and unknown waves to your waves can be some irritative sometimes but sooner it (the energetic waves) takes charge and starts manipulating your waves to sing in same rythem. And so you start feeling yourself lighter and lighter than air. That is the first step in Tantra meditation.
If you are not yoga practitioner then during the different touches on your different parts of body, you may feel anxiety or frequent disturbance in your concentration which may lead you towards ill-fantacies inspired by your own sexual desires. You may want touch the therapist here and there just to feel the sexual arousal but that arousal is accelerated with negative energy which not only to misguide your mind but also fills with filthy thoughts which makes your intellectuality inferior. So you have to keep your devotion on your mind and encourage it towards the calmness only. You have to re-gether your mind and re-concentrate it towards your body. Do it until mind stops turning away. Of course 90 min sessions or 60 min sessions are not to teach you meditation and are good expensive. So it is suggested to do some meditation practice before you start Tantra.
A therapist can only lead you in your tantric journey if you let him/her. And for that therapist need you to surrender yourself towards him/her. Just follow the therapists instructions and s/he can show you the spiritual door within you. Tantra is a divine practice and it helps you to stay away from the malign thoughts.