What you need: a deep cup, room temperature or warm water, an old cloth (anything other than a towel, as fibers from a towel might catch on the nib). We do NOT advise to use any kind of soap or cleaning agent, as these substances may damage or clog your fountain pen nib.
What to do:
1. Fill your cup with water.
2. Remove the cartridge from the pen and leave the nib part in the cup of water over night.
(It may also be helpful to change the water once or twice, if you notice it is saturated with ink.)
3. After the nib is done soaking in the cup, hold it under running water for a few minutes.
4. Dry the nib with a cloth, then insert a new cartridge and assemble the pen back together. The ink flow will be a bit watery for a short time, so it is advisable to write on a scratch piece of paper for a few lines before use.