Sorry the delay in answering.
First I made the sketch in paper (now I use a digital tablet), then I take some photographs of textures that I like, like raw meat, corroded metals.
Then in photoshop with the clone tool I used the textures to paint the sketch.
And I play with the options of the clone tool, for example, I used it in normal to fill the general texture, color dodge for the high lights and color burn for the shadows.