Navigate to Tools > Import > Overview.
Select Customers under the Sales column on the left-hand side.
Browse for and select your file for import and click the Upload button.
Use the drop-down menus to choose the column headings you need for your imported information. If you don't need all the columns, click Ignore in the surplus ones. Then click the Import button.
You'll see a list of the customer contacts available from the file you've uploaded. Select Confirm All to import all the customers on the list. Or use the tick boxes to choose one or a selection of customers only, then click Confirm Import. It's a good idea to uncheck any customers that are already in Clearbooks, to avoid duplication.