Add distilled water, one drop at a time, mixing with a brush until the consistency is a little thicker than bottled ink.
Most problems come from mixing gouache too thick.
Rake the brush across the side of the pen to fill the reservoir. Try a few letters. You can only know if you've reached the correct consistency by writing. When the consistency is flowing nicely, add one drop of gum arabic. This is a binder which will help the gouache adhere to the paper.