A number of different inking systems are utilized by pen manufacturers. Here are instructions for the most common.

Cartridge: Make sure you have the correct ink cartridge. Point the pen away from you and unscrew the barrel. Remove the empty cartridge and insert the new one (push past the initial resistance to break the seal). Replace the barrel and you're back in business. If it doesn't start to flow immediately, give the cartridge a slight squeeze. If it still doesn't flow, provide a small amount of moisture to the nib. This will start the natural 'wicking' process. Be carefull to keep the pen nib pointed away from you and anything which could be stained.

Converter: Converters take the place of cartridges, and are filled from an ink bottle. With a squeeze-style converter, immerse the point in ink and press the converter several times. With a piston converter, immerse the point and twist the knob until the piston is all the way down, then draw it up slowly.