I used the same steps as I did with my Tyranid Termagants. Take a look at that entry to see what I did exactly. Basically, I used Iyanden Darksun to cover the entire model's skin. I tried to avoid painting over the claws and carapace - but it is inevitable that you will get paint onto areas where you don't want them to be. That is why the order in which you apply paint is really important. Start on areas that are large. Also, the Iyanden Darksun paint is thin on areas of the skin in these pictures. I have since touched them up to make sure that the Iyanden Darksun covers the model well.
In these pictures I simply applied the Ogryn Flesh over the Iyanden Darksun after it had dried. Next step is highlighting the skin.