Log in to your Glow account and go to Clients > Live Clients from the main left hand menu. Then click on the name of your client whose website you want to backup.
You will now see the Client Area. From the inner menu, select Backups.
You will now see the Backups screen:
Click on the pink button in the top right corner of the screen that says: Backup now.
You will then see a popup appear in the top right corner, confirming that a backup is now in progress.
Check back in a few minutes and you will see the backup appear in the Backups section further down the screen.