There is a reason that a smooth piece of clay is considered a step. Here is where you must choose which direction your skin texture will take. I'd like to point out that there is nothing random about skin texture. When you select which direction your texture will take- make sure that your decision is justified. Skin texture is created by movement over time, and the skin texture you give to your creation will help inform the viewer how your creature moves.
Step Two: Primary Texture Direction
This is the primary direction that the skin texture will take. This is not to say that all your lines must follow this direction but this direction will be the predominant (and most apparent) direction of your texture. When you lay in your lines try not to make them too uniform, also vary your line thickness to make your texture more interesting and natural looking.
Step Three: Secondary Texture Direction
The secondary texture direction will break up your primary direction and can be much more irregular. Once again remember to keep your lines varied in both thickness and depth.
Step Four: Tertiary Texture
The primary and secondary skin textures are the creases where the flesh folds and stretches as your creature flexes and moves. The tertiary skin texture is the texture of the skin itself. This step is much like the previous two steps but on a much smaller scale and with a lighter touch. The tertiary skin texture should not be deeper than the primary and secondary texture.
Step Five: Smoothing
In this tutorial I'm using an oil based clay (Premo). To smooth out the texture I'll be using a solvent (Mineral Spirits) and a brush, but if you're using a water based clay then you will want to use water. The main point here is to knock down all those hard edges caused by the previous steps. This step is where you get scratches in clay to look more like skin texture. Start out by just using your brush in a stroking and stippling fashion. Then use your solvent in small amounts stroking the surface in all directions but favoring the direction of your primary skin texture. Once the surface looks more like undulations instead of scratches you have finished this step. It is very easy to completely eradicate all texture if you use too much solvent and brushing so practice on a scrap piece of clay before starting in on your sculpt. After you finish with this step your clay may be a little mushy so wait a little before moving on to the next step to allow the solvent to evaporate and your clay to firm up a bit.
Step Six: Finalizing Your Texture
In this step you will go in with your tools and bring out much of the texture that was smoothed away from the last step. You can see that I have accentuated steps 2, 3, and 4 but in light touches all around so as to not lose the smoothness that I got from step 5. If you go too heavy with your detail in this step you may need to repeat step 5. This step is a game of push and pull until you like the finished product.
Step Seven: Finish Smoothing and Adding Imperfections
In this step I use my brush only to help soften step 6. Once I like where the texture is at I add small imperfections by using a toothbrush and solvent. Get a scrap piece of clay and scrub it with your toothbrush soaked in solvent, and flick a mist of dissolved clay over the surface of your sculpt by pulling back on the bristles with your thumb. This adds little bumps and skin imperfections to the finished skin.
This texture is for a creature with very rough and thick skin much like elephant skin. However I'd like to point out that if I were creating human skin texture I would still follow all these same steps, just use much lighter touches all around.