This article covers opening a tab from the "Summary" view. You can also open a tab directly from the "EPOS" view when you have items in your current order. For this See Adding an order to a tab.
To open a tab from the "Summary" view, press the "NEW TAB" button in the top left of the screen.
Type in the customers name and press "OPEN TAB" then "OK"
You will see the new tab appear in the tab list on the left hand pane.