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I study SNRs and I want to learn how sky subtruction is performed on a spectroscopic data since diffuse ISM is known to produce a spectrum which is quite similar to that of SNR, and the therefore contribution of such diffise
emission needs to be properly accounted. if anyone can help me on this subject, I would be greatful.

This done using a spectrum taken nearby in space and time. It depends on the instrument whether a simultaneous sky spectrum is taken. Long slit spectra may or may not have clear sky depending on the length of the slit and the size of the SNR. Multiaperture masks and fibers generally target nearby sky. If you have neither then clearly you should take a sky spectrum before and after with the same setup.When subtracting there may or may not be a need to adjust the wavelength or flux scale; i.e. direct subtraction in pixel space or tweaking (with SKYTWEAK).Yours,
Frank Valdes