1- Make a stone pattern. There can be as many or as little colours as you like, but 2-4 are good enough.
2- Draw parallel lines up and down the bath, first across it's length, then it's width.
To quote an old book, this should "split and elongate the droplets of colours", so the spots are now drawn out into lines.
The pattern as it is now is known as a "Gitgel". It can be printes as it is, but it looks a bit plain on it's own, so it is customary to add a slight shower of white spots . Some books define such a pattern as an "Antique straight" .
~Notes~Some people run the stylus through the trough only once. you can d
Such a pattern is also done with rakes. Once the rake is passed through the bath, it is shifted slightly, so the teeth of the comb are now between the the tracks it has preiously made. The comb is drawn through it again, and the pattern is complete.