Highest, I have not problem telling the process, by the way....that's why I learn so much in here there is plenty of information!!!
With an 5/0 worm hook that was straightened I use the hook point to scratch deep enough every single hair in the primer that is grey color, so after that I use the colors to paint (black and brown) the rat taking care that the acrylic paint goes inside the scratches and removing the outside paint, the white ones are the direct color of the scratch, repeating the process to have the desire result!!!
Hope it helps!!