1. Honest
This is not the time to lie why you have to separate. Maybe this is very strange, but chances are your partner already knows. Communicate the reasons why you split up and mention all the fights.
2. Avoid cliches
Don't say cliché words like, "we still make friends,", "it's not about you but about me,". This will not solve the problem. You tell me the truth why you have to end the engagement without being cliched.
3. Do it personally
This conversation must be done between the two of you. Deciding the engagement and telling it is one of the most traumatic conversations. Choose a safe place, both of you, and don't forget the right time
4. Be sweet
Remember, be nice whatever happens between you two. This will be very difficult. No need to bring and condemn past actions. Don't leverage the past and mistakes. The important thing is to be nice.
5. Return the engagement ring
If you decide the engagement then you must return the ring because the price is expensive. This is the consequence.