A 3-2-1 backup plan keeps three copies of important data, uses two types of storage and keeps one copy off-site.
Map your copies
The working files on your PC are the first copy. An external drive can hold another; an off-site backup service or securely stored drive can provide the third. Make sure the plan covers files outside the usual Documents folder.
Consider deletion and ransomware
Sync alone may copy accidental deletions to other devices. Review version history and retention. Keep at least one backup protected from the same device and account risks as your everyday files.
Test a restore
Once a month, restore a harmless file from your backups and open it. Record when each backup last succeeded, where it is kept and who can access it. Include website and database exports if you run an online business.
Further reading: NCSC: data security.