As of system software 13.2.0, current for December 2021:

  1. Go to the Home screen, and tap System Settings
  2. Scroll to the very bottom, and tap System
  3. Scroll to the very bottom, and tap Formatting Options
  4. Tap on Format microSD card

Every guide I read was either wrong, or missed steps. Maybe an update rearranged things, or changed menu text.

Also remember that SD cards aren’t plug-and-play on the Switch, probably because there’s no clean way in the UI to unmount them.