Before doing anything else you need to make sure that you have removed all traces of old ink from your fountain pen. This can be done by rinsing the nib under cold water for a short while.
You will be required to place the nib or converter into a bowl of cold water, if you are using a piston or a converter pen. Water is dawn into the barrel by twisting the knob at the bottom end of the pen. Turn the mechanism in the other direction if you want to expel the water. Continue to do this until the water inside the ink tank becomes clear.
Use a paper towel to dry off the barrel and nib of your fountain pen. If your pen supports cartridges then use pilot brand cartridge as other brands may not properly fit properly. Again, use paper towel to clean the barrel and nib. Press the cartridge into the nib to make sure it fits into the space provided for the purpose. Make sure the ink is running smoothly. Test the nib of the pen on a small piece of paper if possible.
Immerse converters into the ink bottle for refilling purpose. Again it is recommended to use only pilot branded ink as other brands may have a tight seal. Expel all the air from the barrel by twisting the converter cap. Immerse your pen into the ink bottle to draw up the ink to the desired level. Use a paper towel to remove any ink on the outside of the pen. Put the barrel back into place to start using the pen.