Best way to clean a humidifier properly?

Empty it every day and let the tank dry, that does more than any deep clean. Once a week: unplug, take it apart, fill the tank and base with white vinegar and leave it 20-30 minutes to dissolve the scale, scrub with a soft brush, rinse well. Keep the vinegar off anything electronic. If you want to disinfect as well, do it as a separate step with a weak bleach solution and rinse again. Never mix the two. If it's a wick type, replace the filter rather than trying to clean it. And distilled water cuts the scale and white dust right down.