A CNC can have some powerful emotions linked to it, so slowly is always a good plan. Be sure you are both comfortable with the risk involved first. If you both do. Then my suggestion for negotiation would be to work with the opt in model over opt out. It is far easier to work through a list and say what you would like to happen, what you are ok with happening and to what level over saying what you don't want to happen. Opting out there is always the chance that you could miss something and later regret that, if you miss something with opt in you are just having less fun things and can be added to a potential later scene. It is also a good idea to take your time with these negotiations make sure you are both happy with everything on the list.
Next aftercare for both participants, we often think of the s/ need for aftercare and that is important but in this case the d/ may also wish to have some and that is something that should be on the table with any scene, particularly one that can lead to as powerful emotions as a CNC scene. Better to arrange aftercare and not need it than need it and struggle to get it after, that can lead to some uncomfortable places. So both of you discussing aftercare needs would be a good step.
Finally, safe words. While it may not seem a logical step have safe words. Going without them after years of knowing someone can go well as it can with a first time. But should something go wrong.... that is the exact moment they are most important. Safe words would not make the CNC scene any less impactful but could save from some very bad feelings on both sides of the / should something go wrong. Best case scenario it goes well and they are not needed. But worst case... you can stop things and that goes for both sides of the / it is very possible that the d/ finds themselves not able to continue emotionally and it is best to care for everyone's needs.
If you go slowly, opt in, aftercare planed and safe words in place you are doing everything to reduce chances of things going too far, caring for needs after the scene and ensuring the scene can be stopped if it goes to far. All good things.
Good luck and I hope things go well for you.