Most people go into a breakup conversation with a general sense of what they want to say and no real plan, which is how a conversation meant to be clear and kind often ends up rambling, circular, or accidentally cruel. A little preparation — not a rehearsed script, but a clear sense of the core points — tends to produce a much better conversation for both people.
A useful structure: start with the decision itself, stated plainly, before any explanation. Leading with a long preamble often just extends the other person's anxiety without giving them the actual information they're bracing for. Follow with one or two honest, specific reasons — enough to make the decision make sense, not so many that it turns into a list of grievances.
It helps to anticipate the questions that are likely to come up — is this final, is there someone else, can we talk about it later — and have honest, thought-through answers ready, rather than improvising them under the stress of the moment. If you don't know an answer, “I don't know yet” is a legitimate response, more honest than a reassurance you're not sure you can keep.
And it's worth deciding in advance what you won't do: you won't argue them out of being upset, you won't promise a future conversation you don't intend to have, and you won't let the conversation drift into relitigating the whole relationship. A little structure turns a moment that could go a dozen different chaotic directions into one clear, respectful conversation.