Sketch the torso shape with a simple oval or tapered rectangle
Mark the center line down the middle of the abdomen
Draw the ribcage area above the abs
Outline the waist narrowing toward the hips
Add the upper abdominal blocks as two curved shapes on each side of the center line
Add the middle abdominal blocks below the upper ones
Add the lower abdominal blocks tapering toward the pelvis
Draw the oblique lines along both sides of the torso
Keep the shapes symmetrical but slightly uneven for realism
Use light shading under each muscle segment
Darken the center line and side edges for definition
Erase extra construction lines
Refine the contours and add final highlights and shadows