|You don't have to take the metal ball out. I'm just showing you the marker components.|
This page is full of "as is" paint markers. I didn't fill them with anything special. They are the paint markers (all extra fine or fine.) Top to bottom:
Water based sharpie extra fine
Montana Fine and extra fine
Molotow Fine and extra fine
I color the end of the markers (see the top picture) with whatever color is in the pen. I also label the side of the pen with a piece of tape stating what kind of ink/acrylics it is and the color.
If the ink that you are buying to fill your pens with doesn't come with a dropper or the nozzle won't fit the barrel of the pen, buy a few empty droppers. It will make your life so much easier (all of the inks above came with a dropper or were easy to pour *except* the Noodlers ink. I should've taken a picture of the contraption I rigged up because I didn't have any droppers on hand.)
If you use the Golden High Flow Acrylics, only pour a little bit at a time. If you pour it really fast, it tends to flow quickly and it can spill. Just be careful with it.
Make sure that the inks or acrylics that you are using are fluid like. You don't want to use anything too thick in the pens (regular acrylic paint is too thick.)
One funny story, I bought the empty markers at a few local L.A. art stores. I had the same reaction at almost every store I went to! The looks on the employees faces when they realized what I was doing was priceless. I love watching others get excited about new possibilities with art and art tools!