you will need:
1. not really empty ink cartridges.
2. a scalpel
3. a hot glue gun. hot and filled with glue.
plug in hot glue gun.
Break the marker pen. I broke it off right down to until the nib was protruding on both ends
cut away obstructing plastic at the mouth of the cartridge with the scalpel, this is hazardous.
Attach the nib to the cartridge and add a dollop of hot glue. I found turning the nib while the glue is still molten helps distribute it evenly.
Allow to cool then colour away.
ammends: I thought that if I didn't break off so much of the marker I could put a cover on them to keep them from drying out. You could use the marker's original soft core to draw the ink to the nib. I'll try that next time. I was also thinking you could use any nibs, like brush pens or old fountain pens/ rollerballs, even combine the cartridges at the nib to get different colour mixes...