First take apart the pen and pull the tip out. i used pilot mechanical pencil, better choice will be a pencil with metal case around the tip . the tip will heat up occasionally and may burn the plastic around the tip.
Now cut the plastic isolation of the fiber to fit the pencil tip (be very careful not to brack the glass optic fiber) , thread the optical fiber in to the pencil tip and glue it with a small drop of super glue or epoxy .
As you can see the optic fiber is much thinner then the pancil tip about 50 times thinner. so try to find very fine mechanical pencil and center the fiber in the tip. sorry but i don't have any good idea how to center the fiber.