Step 0: Look for inspiration in art and models you like.
Step 1: Prepare to spend at least two days (assembly + painting) on that single model.
Step 2: Google "how to paint goblin skin" and find results such as this one or this one.
Step 3: Be sure to get a good brush and the paints you need.
Step 4: Paint.
Step 5: Are you happy? Does everything look like it should? If not, repaint the bad parts with black and paint again until happy.