In order to save a submodel to file, open a new window for the submodel, then select the "Save" command on the "File" menu of the submodel window.
The submodel will be saved as a .sml file. In fact, this file will contain nothing to indicate that it is a submodel: Simile makes no distinction between a main (top-level) Simile model saved to file, and a submodel saved to file. If the submodel had influences coming into it from other parts of the model, then the influenced variables in the submodel will be converted to "Input Parameters", indistinguishable from input parameters in top-level models specified by the model designer. The only thing to note is that these automatically-created input parameters will appear red in the model diagram if you have not specified "Min" and "Max" values for them in the equation dialogue prior to saving the submodel.
In: Contents >> Working with submodels