Draw a large oval or rounded shape for the brain outline
Add a slight indentation at the bottom center
Sketch a series of curved, wavy lines inside the outline
Make the lines loop and fold to resemble brain folds
Vary the size and direction of the curves
Add more folds around the edges and center
Refine the outer shape to look uneven and organic
Erase any unnecessary guide lines
Trace the final lines cleanly
Add shading if desired to give depth