Back up your files with Time Machine or another backup method
Sign out of iCloud
Sign out of Messages
Sign out of Music, TV, and Books if needed
Turn off Find My Mac
Restart your MacBook Air
If using Apple silicon, press and hold the power button until startup options appear
If using Intel, restart and immediately hold Command and R
Open Disk Utility from macOS Recovery
Select your startup disk
Click Erase
Choose the correct format and confirm
Quit Disk Utility
Select Reinstall macOS
Follow the on-screen instructions
Set up the MacBook Air as new or shut it down for the next owner