Douro Valley is considered the oldest wine region in the world, and the most famous in Portugal.
It is located in the interior/north of Portugal.
Usually, I would recommend spending some days in Diuro, to make sure you optimize your time there, and before/after, some days in Porto as well.
Normally most international flights land in Lisbon, and from here the journey begins to the Douro Valley area.
Here are the 5 best ways to reach Douro Valley, from Lisbon:
1) Get a Private Transfer Service, starting at only 245EUR per person.
2) Get a Uber from Lisbon, normally starting at 450EUR, but can be more expensive, depending on peak times (recommended UBERXL because of the luggage)
3) Get a train. But there are no direct trains. You may need to get a transfer to the Lisbon train station first. Then take a train to Porto first, and from Porto you would need to change trains again. Please also note that usually hotels in the Douro may be away from the train station, and then you may require a taxi or transfer once arrived in the Douro Valley. The duration should be around 5h to 6h
Prices are around 60EUR Per person for the train tickets
4) Rent your own car, and drive. For some people can be a challenge, specially driving in the city, and parking. Make sure to pay for CDWW to avoid any problems with accidents, crashes, etc. Prices may vary, but normally at Europcar, prices start at 200EUR per day for a reasonable car (sedan), plus insurance, tolls, and gas.
5) By Bus. For those who like travel by public bus, there are several options per day. One is rede expressos. Price may vary depending on the chosen stations, day of the week, company, etc. you may need to get a bus first from Lisbon to Porto, and then in Põrto change bus, and get one different to the Douro. I’ve never done that, would be too much time and exausting.