A squeeze filler sucks the ink into the pen through the nib by reducing the air pressure inside the reservoir so the ink can move in freely. The ink is supplied from an ink bottle and is filled by the user as opposed to ink cartridges which are manufactured and inserted in fountain pens.
Remove the pen lid. Unscrew the base of the pen from the nib. Immerse the pen nib in the ink. Squeeze and release the bulb on the filler several times until no air bubbles appear in the reservoir. Wait three seconds in between squeezes. Remove the pen from the ink and clean the nib with a tissue. Reassemble the pen.
If you wish to replace the ink in the pen with ink of a different color, you will need to clear the reservoir of ink of the original color. Unscrew the pen barrel from the nib. Put the nib into a container to receive the old ink. Press the bulb on the end of the ink reservoir to purge the pen of the ink. Repeat this process until the reservoir is empty. You may then need to clean the reservoir with water before putting in a new color.
Alternative methods for storing ink in a reservoir included prefilled cartridges which are inserted by the owner and thrown away when they run out, accordion-style fillers which use a bellows-like system to draw ink into the pen, blow systems which need you to blow into the back of the pen to depress the sac and suck in the ink, plus many more, some of which are patented by the major fountain pen manufacturers.