No soaking and no boiling. I attach the open end of the carto to one of these (found at the drug store).

It's a dosage syringe for babies. The carto fits snugly inside. I get a bowl of hot tap water and feed water back and forth using the plunger. When squirting water out, I place my finger on the end of the
threads so the water leaves through the air holes rather than the center hole. I do this about a dozen times or until I detect no
juice fragrance from the carto. After that, I blow as much excess water through the carto. To dry, I place the carto in a cool oven for a couple of hours to speed up evaporation. I usually leave the carto in the open air over night to ensure that the polyfill is completely dry. Never had a problem. When the carto gets a really tight draw after a few cleanings, I toss it.