The first time i saw marbling was at a sportsman show in about 1960. A man
was selling a kit (which my mother bought for me) and demonstrating how easy
it was to marble pencils, shoes, etc. The kit was nothing more than a set of
oil paints (thinned), water softener and instructions. The buyer provided
the bucket. Yup, a bucket.
Dip the pencil (or in your case, pen holder) SLOWLY into the cylindical
container, twisting it. The colors will be pulling down with the object.
It's hypnotic. When completely submerged, blow the colors away from the
object to avoid overmarbling when pulling the object back out. You could
even get a production line going by making a rack as in candle dipping.
I've done this also with acrylic paints. No real prep is needed on the
object, but you could experiment.
Good luck and have fun