The easiest way is using a die-cut adhesive tape/film for your cover (or touch screen). You can also attach the cover from behind with screws and then use a rubber gasket, but for low volumes die cut adhesive is pretty easy. You just need to ensure during assembly you use a fixture which will apply a firm and consistent amount of pressure to seal it to the main housing.
You can also seal with more traditional compression materials like foam or rubber. The particular device you posted a picture of uses adhesive, the black masking around the plastic bezel is back printed so you can’t see the adhesive. If you use screws the screw bosses will be visible unless you use a first surface printing technique which won’t last as long.