But you did get the answer.

Select the dress, and click the save material button. In the dialog, indicate you want a material collection. Then save it.

Or is your question how to load images into a shader? If that's your question, assuming you made a color map in a file called foo.jpg, load an Image_Map node connected to Diffuse_Color. Set the file to foo.jpg.

Or is your question how to make a material look like cloth, without regard to the color? That's complicated. It depends on the type of cloth. To fully address that, you have to get down to the specifics of the type of cloth you have in mind. Is it fuzzy like wool or smooth like satin? Totally different shaders in those cases. Is it dull like denim or shiny like leather or silk? Totally different shaders in those cases.

None of your questions have been about shaders, so none of the answers are about shaders.

Your question was how to save a material file. The answer is you click the save button.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)