Most of the world's fountain pens fill using what is called a cartridge/converter system. This means the pen can use either individual ink cartridges or a converter that allows the pen to be filled using bottled ink. Cartridges connect to the grip section of the pen and snap fit into place. The snap sometimes is heard as the seal on the cartridge is broken when pushed into place.
Most converters fit onto the grip section in the same way as the cartridge, although a few will screw into place.
With the converter attached to the grip section, hold the converter at the top with two fingers, just below the twist mechanism. The other three fingers should fan outwards (Imagine a Victorian lady drinking high tea) so as not to get inky from the lip of the ink bottle below. Dip the fountain pen nib into the bottle of ink. You will need to have the nib submerged completely into the ink as the fill hole where the ink enters the pen is located at the base of the nib where the grip section meets the feed section, the apparatus that sits behind the nib.
Twist the converter in a clockwise direction to draw the ink from the bottle into the converter. Lift the pen up from the ink bottle and dab at the nib and grip section with a paper towel or tissue to remove excess ink.
Check to make sure there is a tiny air pocket in the chamber of the converter. If the size of the air pocket is large you may expel the ink back into the bottle by placing the nib back inside and twisting the converter knob counter clockwise. Repeat filling instructions and the converter should have a greater fill of ink. Reassemble grip section to the barrel of the pen and enjoy writing!