1. Open developers.facebook.com/apps and log in with the Facebook account that runs your ads. Tap Create app, choose the option for managing ads with the Marketing API (or Other → Business), give it any name, and create it.
2. Open the Graph API Explorer. Pick your new app under Meta App, keep User Token, and under Permissions add ads_read and ads_management. Tap Generate Access Token and approve.
3. That key only lasts about an hour. To make it last 60 days, copy it, open the Access Token Debugger, paste it, tap Debug, then Extend Access Token at the bottom. Copy the new key.
4. Paste it above and tap Connect. The app warns you a week before it runs out, and you repeat steps 2–3 to renew.

Want one that never expires? In Business Settings → Users → System users, add a system user, give it your ad accounts, and generate a token with ads_read and ads_management and expiry set to Never.