To reach Villasimius is recommendable to go by car. In  Cagliari airport and in Cagliari harbour as well you have several rental car 

To reach Villasimius from the Cagliari Elmas airport clik here, from the harbour clik here

Once you are in the central square of Villasimius follow the Via del Mare with direction Capo Carbonara; at the first crossing turn to the right and go on for about 1 km following the indication for Camping Spiaggia del Riso. Just 20 meters before the cross for the camping turn to the right and take country track that enter in a pine forest, after about 500m turn to the left on the first cross ing and take Via dei Limoni, the first villa you’ll see is Villa Mirta!