When the pen is new, first wash the blades/nib with soap and hot water to remove the protective oil that is applied during manufacture.
Fill the gape between the blades/nib with paint by either brushing with a brush, dripping it with a pipette or dipping the nib into color.
Automatic Pens are developed for use with water-soluble inks, but work with most kinds of paints as long as the consistency is similar to milk. If you want to use permanent inks, be sure to clean them carefully after use so that the blades do not dry together and get ruined.