Assume the position.
How you line up your foot, toes, and heels will not actually vary your serve so much, but for Venus, she keeps her back foot almost parallel to the line and points her front foot toward the right post of the net. She likes to point her toes at the post because this angles her body, positioning it in a pose that will generate torque. Really, though, this is just her style.
"John McEnroe, his serve is completely different – a great serve, though, but he was completely sideways."