Listen to cookies shading tips, but here are some techniques if you dont know them.
Hatching: diagonal lines that keep going much like a /// or \\\, the closer and darker they are, the darker the area, the brighter and spaced apart they are the lighter the area.
Cross-hatching:Can be used for a bit more detail or darkening even further. Cross the diagonal lines something like an X.
Shading: Use a straight grey color, same pressure, and all of that. A bit harder to master since you must havea steady hand to keep the same color.
I suggest doing the shading like painting. Layers. on the first layer do it a lighter tone. Go back in the same area, but do it a bit darker half-way through, and then, for the extra details go really dark.