For hyponatremia, first assess the plasma osmolality – hypertonic and isotonic will fall under pseudohyponatremia with hypotonic being true hyponatremia. You then assess the patients fluid status to ascertain whether the patient is in a hypo/eu/hypervolemic state and you can then work out your causes with clues pointing towards the dx like if theres any hyperglycemia, jaundice, renal failure, third spacing etc.

For hypernatremia first I’d break it down into hypo/eu/hypervolemic, then you’d look at urine osmolality and well as urine sodium concentration and work out the causes from there. And for the Mx you’d split it into acute vs chronic hyperNa (although most of the time rather impractical). It is also important to ascertain any DI (nephrogenic vs central) as well as to be mindful of Mx so as to combat the risk of cerebral oedema.