Begin by taking the lid off the pen and unscrewing the handle. Next you'll want to use the paper clip thing or whatever you can to get the black plug...
Having wanted to expand my ink marking tools I decided to buy a brush pen, I bought the Pentel brush pen and tried it out on the Bristol Board I usually work on and it was all good. However, several days later I did a smudge test and behold it smudged the hell out of it, also large areas of black were streaky and not consistent. I realised soon after that the ink was 'india ink like' and not the real deal. I like the way the brush works with the tapering lines and the easy strokes but the ink was useless, what was I going to do. . . except CREATE MY OWN.
I had a few watercolour water brushes lying around, they are like a regular brush with a hollow handle to put water in and squeezing it pushes water into the bristles. You know the sort of thing. They can be picked up in art stores or online for cheap too.
Begin by acquiring the right materials:
A Watercolour Brush pen (popular brands are the ZIG Watercolour brush)
A paper clip or similar
Piece of paper