Jif Waterman of France invented the cartridge. Buy an ink cartridge, unscrew the section from the barrel. Push the ink cartridge into the section. Screw the barrel back onto the section and write. If a converter is used, simply unscrew the barrel from the section, dip the nib into the ink well filled with ink, and turn the converter knob at the end, thus sucking up ink into the converter.  Turn the converter in the opposite direction to force a few drops through the nib back into the bottle, screw the barrel back onto the section, and after wiping the nib, write with the pen.  The majority of pens today are cartridge fillers (since the ‘60s).  And, if they are cartridge fillers... they can use a converter (which is recommended, less waste of plastics, and broader variety of colors). Most converters are pushed into the section of the pen, much the same way you would insert a cartridge. However, Montblanc converters, once pushed into the section and onto the nib, they are then screwed in securely.