When we learned Nepali in School the letters श transliterated as Ś and ष transliterated as Ṣ was either pronounced same or ष was pronounced as ख.

In the below two words the transliteration is different.
ऋषि transliterated as ṛṣi vs शीतल transliterated as śītal

So how are we actually supposed to pronounce these two letters differently?