Open the file in Sketch and go to the page named ‘Artboard’. Build out your interface here by inserting the UI elements you need from the ‘Bars’, ‘Controls’ ‘Sheets & Alerts’ & ‘System’ layers. You can also drag in elements from the Sketchs ‘Components’ menu. If you need to edit buttons, labels, icons or colors, use the overrides provided in the Inspector panel. If you need to edit a symbol beyond what is possible from the overrides, then right click and ‘Detach from symbol’. If your app requires multiple screens, copy the artboard for each screen you need to design. To use the UI kit as a Sketch library, go to ‘File > Add As Library’. The UI elements can then be inserted as symbols from the insert symbol menu.