Draw the head as an oval, with the chin pointed and the top of the head more rounded.
Move the pencil down from the head to the neck. The neck will be more slender than the head, and will flush outward at the shoulders.
Move the pencil downward to draw the arms at the edge of the shoulders. Pose the arms somewhat bent (a common mannequin pose, seemingly for no particular reason).
Move the pencil down to the hands, which will not have individual fingers. Draw the hands like extensions of the arms; they will be thinner at the wrists and thicker at the palms.
The fingers will look like one large finger, bent all the same and somewhat more narrow than the palms, as if the hand were wearing mittens. Although many mannequins do have individual fingers, it is acceptable to draw the hands without them, and will make the mannequin more fake in appearance. Then, complete the arms by moving the pencil back up the other side to the armpits.
Draw the body, narrowing the width gradually until just above the hips, then moving out again to widen at the hips.
Move the pencil down the outside of the legs, which will be bent to represent little or no motion. The legs must be straight enough to keep the mannequin upright and stable. Draw down to the feet, which will be posed slightly outward to balance the mannequin, then draw back up the inside of the legs until they meet.
Shade one side of the face to give it a rounded appearance, so that one side is light and the other side is darker, becoming darker still toward the outer edge of the face. Imply a mouth without expression by drawing a single line where the lips would be. Do not add hair or eyes.
Draw the clothes to fit over the body. What you choose to make the mannequin wear is your decision, and depends partially on the gender of the mannequin. Note that the clothes will be only slightly larger than the body underneath, and will be only slightly rumpled.
Draw a horizontal line behind the mannequin's body, about one quarter of the way up. This will represent the back wall of the display case.
Draw a rectangle around the exterior of the mannequin to represent the display window. Imply the bricks of the building by adding small clumps of rectangles in the area around the window.
Draw a horizontal line beneath the display window, representing where the window meets the sidewalk.