Uploading a template for sale on Mediamodifier is simple, most likely you won't need to make any major changes to the Photoshop file. However here are a few formating rules that must be met before a template is ready for uploading.
There can only be 1 smart object placeholder in the entire PSD file.
Example: you have a business card mockup with 2 placeholders, 1 for front and one for the back side - unfortunately this kind of template is currently not supported. However multiple instances of the same smart object are supported. This means you can create a template where 1 placeholder image is duplicated many times.
Make sure the PSD does not contain any text layers.
Make sure you rasterize all text layers. Missing fonts can cause error while producing an image from the template.
The smart object cannot be inside a folder.
Simple move the smart object out of all folders. This only affects the smart objects, all other designs element in the PSD can still stay inside folders. NB: the smart object does NOT need to be the top layer in the PSD file, you can add layers or even folders above the smart object layer.
Hide all layers inside the smart object.
Once you are ready to upload your PSD template, simply hide all the layers inside the smart object by unticking the eye symbol in the layers menu. This step ensures that no placeholder or dummy images get added to the full image.