That’s the catch.
Breaking up with someone (unless the feeling is mutual always sucks)
The obvious rules (i.e. face to face is most ‘noble) apply but you know your partner best.
e.g. With a particular partner of mine we always spoke on Google Hangouts more than face to face so we sometimes discussed difficult things there
Just do it on the medium that you two are accustomed to communicating
Beyond that. Be honest. Don’t lie. And say it quickly and don’t be the person to provide sympathy.
That can’t be you