First create the case, select the round rectangle tool then set the radius to 40 px. In a new layer create a large #2B282F rectangle. In this tutorial the size of the case is 430 px x 250 px.
Once you make the first rectangle, edit the round rectangle radius setting to 20 px and draw another rectangle with the same width. We want the the top corners from the second rectangle.Note: picture guide below is resized to fit the tutorial window.Step two: