HOW TO ADD OR REPLACE INK IN YOUR TWIST MECHANISM PEN...
With so many twist mechanism pens available, there are multiple ways to replace the ink.  Some twist pens have a nib that will unscrew to access the ballpoint cartridge.  Make sure you do not lose the spring either attached to the ink cartridge or it can be found at the bottom of the barel.   

Other twist mechanism pens have you pulling off the twisted mechanism at the top of the pen.  You might need to unscrew the twist mechanism to find the ink cartridge.  When place the the op of the pen back on the barrel of the pen, make sure it is properly on before pressing down on the top of the pen.  Never forcefully push or hammer a part onto the pen.  This force may cause damage to the pen.

 
BALLPOINT CARTRIDGES
Most twist mechanism pens take ballpoint cartridges.  You can order extra cartridges online.  A common and high-quality brand I use is the SCHMIDT easyflow 588 ink cartridge.

 
MATERIALS USED IN CREATING HYBRID FOUNTAIN PENS...
The wood used is mainly Padauk, Cocobolo, Black Walnut, Maple and Rosewood.  I pour my own resin blanks and use a variety of  pigment powders to produce vibrant colors and swirls.