I don´t think there is a straight-forward answer for that. I guess you set up the ragdoll itself using (limited) revolute joints and then... tinker around with it?
You´ll have to apply forces to the limbs then in a specific way as to create anything that resembles a walk. You probably also can apply an upwards force to the head to counteract gravity so the creature doesn´t constantly topple over. Other than that... I might give it a try, it sounds kinda fun to figure out.