This one is pretty easy. Just wait at the bus stop in Dominical for the bus heading to Quepos. Once in Quepos take the bus to Jacó.
Option 2: More expensive but a little bit faster: take the bus from Dominical to Uvita. Get to the Pulmitan station and take a bus headed to San José. You will have to pay full fare (about 6500) and the driver will let you off on the side of the highway headed into Jacó. From here you can take a taxi or walk the 2km into town. From Uvita to Jacó (highway) is a little over 2hrs.