To use simply open the PSD you wish to use as your mock-up, double click on the red coloured layer, also named "EDIT ME" and you will open up a new canvas which you will be able to save your image/graphics onto. It will not ask you where to save once you have completed, just save it. Once you have saved it go back to the main PSD mock-up you previously opened and you will see that it has been updated with your own image and graphic!

No need to tweak anything else, all the reflections, lighting has been adjusted to the smart object itself to match its scene!