How do I set an out-of-office automatic reply in Outlook before I go on leave?

In new Outlook and the web version: Settings (gear icon) > Accounts > Automatic Replies > toggle it on. Set a start and end time so it switches off by itself when you're back. There's a separate box for 'Send replies outside your organization', tick it and write a different (usually shorter, less detailed) message for external senders. In classic desktop Outlook it's File > Automatic Replies (Out of Office). One tip: name a colleague as the contact for urgent stuff, and always set the end date so you don't return to it still running a week later.

Set the start AND end time, that's the bit people skip. Then it turns itself off automatically and you're not still auto-replying days after you're back at your desk.

Use the 'inside vs outside my organization' split. Give colleagues the detailed version with who to contact, and keep the external one short and vague. No need to tell the whole world your movements.

If you're on a personal Outlook.com account rather than a work one, it's the same Automatic Replies setting but there's no inside/outside split. That's a Microsoft 365 work-account feature.