You need to be clear that you want to pursue and maintain a sexual relationship, rather than reconcile. Also, make it clear to him that you have no interest in reestablishing this romantic relationship with him. Let love be something you both are interested in this FWB relationship. Keep the relationship clean, pure and honest. You just need to remember that this is a relationship that has nothing to do with feelings.
Calm you down before entering into this NSA relationship
If you decide to establish this FWB relationship with your ex, you need to give you enough time to calm down before you formally enter into this tinder free relationship. Even if you break up amicably, you need to wait a few weeks before visiting your ex. But if your ex is forced to break up, you may have to wait six months or a year before you can invite her into the NSA relationship. Otherwise, in her fantasy of you has not completely stopped, your sudden appearance and disturbance will let her rekindle the hope of reconciliation with you. If you don't really intend to do so, your actions will undoubtedly hurt your ex. So the best way is to wait until you two have no emotional needs for each other and talk about your physical needs.
Express your intentions explicitly
Above all, you should tell your ex the purpose of your visit directly. You have to tell her that you want her back this time to have sex with her. There is no other plan. At the same time, you should suggest that you are dating other adult friends, which can dispel her fantasy or avoid her misunderstanding of your intention this time. In short, you should let him understand that the only thing you are interested in her now is sex. When you feel like you're beginning to have a possessive interest in your ex, you should end the relationship immediately.
Don’t stay with your ex all night
Staying with your FWB is not allowed in an NSA relationship, let alone the FWB is your ex. When you spend the night at her house, she will make breakfast for you the next morning, if she wants to make up with you. To avoid this unnecessary hassle, when you end your sex life, you should pack up and go home.