Very easy to use. Editing the smart objects is really simple just double click on the green layer and change the design. Save this PSD file and that is it. Go back to the original file and your design is shown. Please note that the psd files are layered with a Background (the image) and a Smart object layer for you to insert your art. The props are on the same layer.

https://creativemarket.com/Bobboz/1234353-Homemade-Cookies-Cards-MockUp-Set