Be definitive. Don’t beat around the bush. It sucks, it hurts, but dragging it out or giving them hope of continuing the relationship just makes it worse for everyone. Make sure you are sure with the decision as well
To go with this, never go back. Once it's done, move on yourself. The memories will likely skew toward the positive as time goes on. That's ok, bit remember you broke it off for a reason and let go.