There's a round toner cap hidden inside the side panel of the cartridge. Looking at the cartridge with the foldable handle facing you (the green drum side facing away), go to the right side and remove 2 screws. Slide that cover off along the post that protrudes out a bit. Under that cover, look for a round plug the size of a nickel. If you have strong fingernails you can slowly pry it out. Otherwise use something like a credit card to wedge it out. A metal screwdriver can gouge the plastic, so be careful. Once the plug is off, pour in toner w/ a funnel. Easy!