I have purchased mock up designs off of etsy. When I open the file, there is a smart objects layer that when I double click it, I am supposed to be able to place my image there. Do I have to have the design completed BEFORE I use the smart object?
There is no standard Mockup design. How a mockup is used depends one the Mockup PSD/PSDT... We would need to see your Mockup Layer palette and may also may need some details about some of its layers. Some Mockup I have seen on the web are very good others no so good.