Free Product

What is the rules under gst for free product for example I am purchasing oil and sugar but to increase sale I have given scheme as free sugar on 5litre oil purchase so in this case I am taking value of oil and sugar is given free of cost in this case what is the procedure for billing and selling sugar in under cost purchase it with value as gst doesn't allow to sell under cost loss selling.

what all documentation needed if selling as free with oil if needed.