Start with what cannot be replaced
A new computer can download most apps again. It cannot recreate the photos you took, the documents you wrote, or the notes that explain an unfinished project. Make a short inventory of those files and the devices that hold them. This simple list catches the folders that an automatic backup often misses: a camera card, a phone export, or a desktop folder that grew outside the usual documents directory.
Keep copies in different places
A practical arrangement has a working copy, a local backup, and another copy away from home. The second location protects against more than a broken drive; it also helps when a device is lost or a room is damaged. Synchronization is convenient, but a mistaken deletion can synchronize just as quickly. Keep version history for the folders that change often, and make sure at least one copy is not always connected to your computer.
Practice the return trip
Once a month, choose an old photo and a recent document. Restore both into a temporary folder and open them. Check that the file names, dates, and contents survived. A successful backup notification tells you that a process ran; a small restoration test tells you that your work can come back. That is the promise a backup needs to keep.