As always I recommend Chest of colors tuts
http://chestofcolors.com/tag/osl/
(care, there's no excerpt, all content following with a massive vertical content. I'll recommand the "c'mon baby light my fire" tut
Did mine a while ago (was my 1st OSL, in hindsight it lacks subtility) a look like this
Take some time but it's not that hard with lahmia & patience
Standard materials (color 1/5 & lahmia 4/5) make successive layers and you're good to go
Metals & reflectives : prefer inks