After you select a backup disk, Time Machine immediately begins making periodic backups—automatically and without further action by you. The first backup may take a long time, but you can continue using your Mac while a backup is underway. Time Machine backs up only the files that changed since the previous backup, so future backups will be faster.
To start a backup manually, choose Back Up Now from the Time Machine menu
in the menu bar. Use the same menu to check the status of a backup or skip a backup in progress.