We recommand to rent a car, it will be cheaper than a taxi.
A taxi Jerez - Zahara would port you between 100 € and 110 €
There are no good public transport.
If you come from Jerez or Sevilla go to Barbate and then to Zahara.
Coming from Malaga o Gibraltar go to Algeciras, then the N 340 direction Cádiz until the juntion to Zahara.