What is the proper way to handle a cartridge? 

As soon as you remove the cartridge from your printer, place it into a plastic storage clip. This will protect the cartridge and prevent it from drying out. Bring the cartridges to the Costco Photo Centre for refilling as soon as possible after removing it from the printer.
If you do not have a clip, place one to three cartridges in a letter-sized envelope. Caution: Do not rub print heads together when transporting cartridges. Never touch, clean or place tape over the print head nozzles. Be sure to store all cartridges in a cool place.