Driving East on the main street, #19 Benito Juarez (the same street as Hotel California), go through the intersection at Calle Melchor Ocampo where you'll see the 1st sign on your left ~ and keep going on the single lane highway.
Less than 2 minutes later, you will see a 2nd sign on your right (same sign but with the arrow pointing left). Turn left at the next (sandy) road and then pass through the white arches. Spa California will be 300 ft. on your left.
Although this map shows other possible ways to get here, only the directions above will work.
Due to the single lane highway, Spa California is not walking distance from town. If easier, Javier can also arrange for a taxi to pick you up and then return you to your home/hotel (the cost from downtown Todos Santos is approximately 100 pesos each way).