Start by blocking in all the shadow areas. From here you can start to divide the shadows into light, medium and dark. Remember, you sometimes put in shadows and tone and blend it back with your rubber.
Start with a hatching stroke using a 2b pencil and graduate to a 5b in the very dark areas.
You can see that light is coming from the right side so the darkest shadows are on the left side. The musculature and spine are defined by the graduated softer shadows.
Apart from diagonal hatching lines you can also use strokes to convey shade and form, eg. around the legs and arms I've used some semi-circular strokes to enhance the roundness.
To soften and blend you can use your fingers but not too much as this can lead to an insipid effect.
Keep the edges of your shadows soft to convey the flesh.
Until next time, enjoy.