Remember that while this create sexual stimulation, it is not intended as a sexual practice.
The intention of tantric massage is connection, not necessarily a sexual practice. You should be able to create that sensual mood by aligning the different energies of your bodies and minds.
After a massage, your partner should feel relaxed and peaceful. Unless you are both comfortable with it at that moment, you shouldn’t massage any of your sexual organs.
In fact, in the beginning, while you are performing this as an exercise in connection, you should be engaging in casual tantric massages. As you get closer and more intimate, then you can connect in a more sexual way while you are practicing these techniques.