Back up your data with Time Machine or another backup method
Sign out of iCloud
Sign out of Messages
Sign out of Music and TV if needed
Unpair Bluetooth devices if desired
Restart the MacBook Air
On Apple silicon MacBook Air, press and hold the power button until startup options appear
On Intel MacBook Air, restart and immediately hold Command and R
Select Disk Utility
Choose Macintosh HD
Click Erase
If available, erase the volume group
Quit Disk Utility
Select Reinstall macOS
Follow the on-screen instructions
Set up the Mac as new or shut it down after installation for the next user