I get the NC rule. I agree and think it’s good but I noticed the rules don’t mention anything about letting your ex know that you are about to cut contact. Is ‘no warning’ part of the point? I’m concerned because I did this to my ex once before- before we started dating- I told him i couldn’t talk to him because I was already seeing someone else and it really hurt him and I didn’t know it so I don’t want to remind him of that. Should I say ‘I’m taking some space in case you don’t hear from me for a while’ ? Should I set up the NC Rule before doing it or give no warning? THANK YOU!
Its best not to tell them that you are not contacting them.
If they keep trying to reach out to you during this time. (Like multiple times spread over multiple days) Just tell them that you need some time and space. Keep it at that, don’t get into any reasons.
It is mentioned. No warning 
Should I tell my ex that I am doing no contact?
Ideally no. You want them to wonder what happened to you and why you are not contacting them. You want to be on your ex’s mind as much as you can. And telling them you are not contacting for some time will defeat this purpose.
However, if your ex is currently calling you everyday or texting you everyday, then yes you should let them know that you don’t want them to contact you for a short period of time. Don’t give them any specifics. Just tell them to not contact you until you decide to contact them. Let them know you need some space and time right now.
To everyone: Thanks for clarifying. It really helped a lot.