In your web browser address bar, login to Outlook on the web https://outlook.office.com

Once logged in choose the 9 dots at the top left to display the applications.

Choose Sharepoint.

Under the following section, choose the sharepoint you want to sync.

Choose Documents.

In the document tools bar you will see the sync icon, click this

You may see the following popup on Windows, click open link

If you are not already logged in to OneDrive you may now be prompted to login. Once logged in the says will start but may take a while to show and complete. Once completed add any additional sites you may require.