Take the pen lid off.
Unscrew the barrel from the nib --the pointed end of the pen which distributes the ink on the writing surface.
Insert the ink cartridge into the portal on the nib. Push the cartridge in until you hear it click into place.
Screw the barrel on over the cartridge. Replace the pen lid.
Unscrew the barrel from the pen base. If the pen has a lid then take of the lid and unscrew the barrel from the nib housing.
Insert the disposable ball point into the barrel or the nib housing. Slide the spring over the top of the ball point if loading into the barrel.
Screw the barrel back onto the pen base or the nib housing. Replace the lid if necessary.