How do you let someone go well, as a first-time manager?

Firing someone well starts weeks or months before the actual conversation, not in the conversation itself. If someone is surprised when you let them go, that surprise is usually evidence of a process failure earlier — unclear expectations, feedback that was too soft to actually register, or both.

What should happen before it ever gets to this point

Specific, direct feedback about the exact gap, given early enough that the person has a real chance to close it, with a clear understanding of what "closing it" looks like. If someone hears "you're doing fine, just keep improving" for two quarters and then gets let go, the actual failure happened in those vague conversations, not in the final one.

The conversation itself

Be direct in the first sentence. "I need to tell you that today is your last day" is harder to say than "so, how are things going," but it respects the person more — it doesn't make them sit through preamble wondering where the conversation is headed when they likely already sense it.

What to actually say, and what to skip

State the decision, state that it's final and not up for negotiation in this meeting, and move quickly to practical matters — timeline, severance, references, logistics. Long explanations or justifications in the moment rarely help the person and often make you feel better at their expense. Save the "why," in detail, for the conversations that should have already happened.

The part almost every new manager gets wrong

Continuing to manage everyone else on the team as though nothing happened. A team notices when a colleague leaves under these circumstances, and addressing it directly — without violating the departing person's privacy — does far more for the remaining team's trust than pretending it didn't happen.