Isopropyl alcohol (99% is the good stuff), maybe 2-4 cotton swabs, air dry. Dip swab, rub contacts softly, flip sides of cart, rub, cycle swab, repeat. I had no idea for so many years I was actually damaging the cartridges by blowing into them which just put hot air and moisture on dust. If you have any Switch cartridges or older, and haven't done a proper cleaning, it's amazing how shiny and vibrant the contacts will end up looking, along with the cartridge working more stably.