Remove the cap of the water-based marker.
Use a box-cutter knife to cut or level the top from the marker. Some markers have a detachable nib that is easy to remove, while others will have to be cut off.
Mix ink or paint with small amounts of paint thinner until you get the consistency you want for the kind of writing or drawing you'll be doing.
Use a small funnel to fill the body of the water-based marker with ink.
Seal any leaks in the marker with a water-based putty. Alternately, sticky tack, hot glue or waterproof caulking will work.
Create a nib from felt by plugging the top end of the marker with the fabric.
Use scissors or a box-cutter knife to shape the fabric nib into the shape you want.
Turn the marker upside down on top of a piece of scrap paper to let the ink seep into the nib. Once ink starts to bleed onto the paper, the marker is ready to use.