One good way to go is simply to sit in front of each other cross legged and have a short meditation minute.
You can do this with open or closed eyes.
Synchronize your breathing.
This gives you the possibility to refocus your energy and create space in your mind.
Join your hands in front of your heart or third eye chakra in a prayer position.
You can as well express your wishes for this session internally or verbally to your partner.
You can say for instance: "I want to invoke the tantric sex spirit and infinite love - I want to give my partner immense pleasure and bring him (or her) to absolute bliss - I want our energies to merge and be one..."
This is of course just an example.
You can spontaneously say your own invocation.
You can as well ask for guidance or help.
Again, this can be done internally or out loud.
You can speak in your own name or in the name of your couple.