Set the scene. Don’t break up at your friend’s place or at a social gathering. Find a quiet place where both of you feel comfortable and safe.
Don’t use a phone. It is not mature to send an SMS or leaving a message on the phone that the relationship is over. Dealing with negative situations is a life skill that you have to acquire sometime, and the sooner the better.
What to say. Don’t accuse your ex and use this as an opportunity to point out everything your ex ever did wrong. There are always two sides to an argument, and breaking up is hard enough without turning it into a free-for-all wrestling match. If you just focus on the nit-picky details, your ex might try to find ways to claim these are your own fault. Rather explain your feelings for him and how they have changed.