Heat up 5 litres of water in a metal bucket to almost it's boiling point. Remove the bucket from the fire and add one kilogram of caustic soda (this operation must be done in a ventilated place because of the irritating vapours). Put the bucket back on the fire and stir in order to dissolve completely the soda in the water. Add little by little the 5 litres of oil stirring the mixture continously in order to make it homogeneous.Little by little this mixture will become a creamy yellowish colour, continue stirring taking care not to change the direction or speed, otherwise the mixture could curdle.The process is finished when the soap becomes thick sticky, approximately 30-40 minutes after starting this operation. At this point empty the mixture out into a wooden drawer or mould that has been previously covered with a coarse paper. Leave to settle for one day.Once dried, cut it into portions and remove from the drawer. Keep them in a dry place for 8 to 10 days.