Asklemmy

Don't be mean, or shitty about it, but be absolute. "This relationship is no longer working for me, and I cannot continue to see you anymore." Not, "IDK maybe we shouldn't see each other anymore." Whatever variation of how you say it that works for you in fine, but no wishy-washiness.

Cut all contact after. It's very rare for people to be able to be friends with an EX. And it's even harder in the immediate time after the breakup (that's how you end up back together) I say this as someone who is friends with multiple EXs. Those friendships took time, and space, immediately after the breakup.

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

Even during the breaking up, try not to console them or make them feel better. I don't mean be a jackass. Other comments have covered the overall approach very well already. I just mean trying to help them feel better about it is really just for you. They're going to and are allowed to process it however they're going to, and you give up a right to try to be a part of their healing when you break it off.

Be kind during, give them space if you actually want to remain friends after, and let them process and heal how they will.

As I've gotten older I've actually preferred to be broken up with over being the one breaking it off. It's hard to do that to someone you care about but isn't a good fit for whatever reason. When it's broken off with me I can say so be it and start moving on and not have to hurt another person in the process.

Whatever you do, don't just stop reaching out one day. Be mature enough to communicate, and kind enough to say hard truths about why.