Start off by unlocking the Background layer of your image. Do this by double-clicking on it (to skip the Create New Layer dialog box, hold Alt/Opt while double-clicking).
Go to Edit>Transform>Warp and drag the bottom right corner point on your image. Move the corner inwards to the centre, and pull the Warp area out of the canvas area using the black points. This creates more of a fold.
Hit Enter to apply the changes and then go to New>Place. Open up your image from Place and drag its layer under the first image in the Layers palette. Press V for the Move tool and position the image underneath.