Select the white or blue cloud icon in the Windows notification area, and then select Help & Settings, then Settings, then Backup tab, then Manage backup.
In the Back up your folders dialog, make sure the folders that you want to back up are selected. If the folders all have a link that says Stop backup, you are already configured to have those folders backing up to your OneDrive. If not, please continue.
Select Start backup.
You can close the dialog box while your files sync to OneDrive. Or, to watch your files sync, select View upload progress. If you already closed the dialog, to open the OneDrive activity center, select the white or blue cloud in the notification area.
This content was copied and revised from and may be outdated if there was an update to OneDrive. Current steps, as well as additional information can be found at: https://support.microsoft.com/en-us/office/back-up-your-documents-pictures-and-desktop-folders-with-onedrive-d61a7930-a6fb-4b95-b28a-6552e77c3057