The cap on the Potters Pen unscrews OPPOSITE from most other things in the world. You want to turn it CLOCKWISE to loosen, not counter-clockwise.
QUESTION: HOW DO I USE THIS THING?
Give the pen a thorough shake before you get started. To lay down an nice line, you need to depress the pen tip as you give the bottle a little squeeze. If you find it's not working very well, or the tip is hard to depress, or you are having to squeeze unexpectedly hard, try cleaning the tip (see below). When it is working correctly, it lays down nice lines and is relatively effortless to use.
QUESTION: IT'S NOT DISPENSING WELL. HOW DO I CLEAN THE TIP?
It seems that the underglaze or resist will settle into the mechanics of the tip and dry out, clogging the pen. Here's how you fix it.
Hold the pen upright with the tip/cap up (so it will not spill)
Unscrew the cap CLOCKWISE (the opposite of normal)
Put the pen body in a cup or a mug, or some other container, so it will not tip over and spill.
Submerge the cap in a glass of water and let it soak for a few hours
After soaking, put the cap under running water and rinse while depressing the tip. Continue until the water rinses clear and the tip moves freely.
Install the tip/cap on the pen body.
Give it thorough shake and then give it a try.
If it works, but the liquid seems a little thick, you can add a little water to thin the underglaze.
QUESTION: MY POTTERS PEN IS EMPTY AND I WANT TO REFILL IT.
Just remove the cap, refill and put the cap back on. Remember that the cap unscrews CLOCKWISE (the opposite of normal).