select the model you wish to apply the surface to by clicking the root of the model in the content list or by selecting it with the keyboard (CMD+SHIFT+T [win] CTRL+SHIFT+T [mac])
Select the surface you wish to apply the shader to (RMB over shader, then Select Surface…, or CMD+SHIFT+M [win] CTRL+SHIFT+M [mac])
apply the shader from your runtime library, likely this is in Content/Shader but not necessarily
If the shader does not modify the “default” shader structure on the surface; meaning your list of surface options didn’t change when you applied the new shader you can copy and paste that shader to other surfaces. If the shader did change that “default” structure you will need to repeat applying the shader in the method I explained above.
that’s the VERY basics of this.