Take the direct bus out of Samara heading to San José. You will arrive at the 7-10 station. The bus that heads to Dominical leaves from the Tracopa Station. Once you arrive, head to the Taxi stand at the side of the station – you will see it, it is labeled and there is a person there to match you with a taxi. The taxi should charge around 2000₡ – 2500₡. You have every right to ask the driver to run the meter for the trip rather than a set price, but make sure you make it clear this is your preference right at the start.
To do this trip, you have a couple of different options. If you take the [very early] direct bus at 4:30am you can then grab a bus down to Quepos and then from there grab another bus heading to Dominical. The schedules are on each page – there are several. You should be able to get to Dominical by mid day.
If you are leaving Samara on a Sunday then you can take the Direct bus at 9am. Then wait about an hour and take the direct bus to Uvita. Of course you can also head to Quepos at 2pm if you get to the Tracopa station in time for that departure, but it would make more sense just to grab the direct bus to Uvita