How do you end a relationship well?
Clearly, in person, and once — because the kindest-seeming approaches, softening and staying in contact, reliably produce more pain than a clean ending does.
What doing it well involves:
Being certain first. Ending a relationship to provoke a change, or while genuinely undecided, causes far more damage than waiting a week to be sure.
In person, where safe, and privately. Doing it by message is defensible only where distance or safety makes it necessary.
Saying it is over, not that it might be. Ambiguity is experienced as hope, and hope extends the pain. The single most common cruelty is being unclear in order to feel kind.
Giving a reason, briefly, without a comprehensive inventory of faults. The other person deserves to understand and does not need a prosecution.
Not negotiating it in the moment. You can listen without reopening the decision.
Letting them react. Anger, tears, questions and silence are all reasonable, and sitting with it is part of the job.
Choosing the timing with some care — not before their exam, not at a family event — while accepting that there is no good time and that waiting indefinitely for one is avoidance.
Afterwards:
No contact for a period, which is unkind-feeling and is what actually allows both people to recover. Checking in is frequently for the leaver's guilt rather than the other's benefit.
Do not continue sleeping together, which prevents either person moving on and is a reliable source of further hurt.
Handle the practical separately and promptly — belongings, money, shared arrangements, pets — rather than letting them become a reason for contact.
Do not disparage them to mutual friends.
If you are being left: you cannot argue someone into wanting to be with you, and attempting it damages your standing with yourself. Asking for a clear answer is reasonable; asking for another chance repeatedly is not.
Where children, finances or safety are involved, the process is different and support should be sought.