This is one of the most famous trekking trails in Yilan.There are always some people on the trail, but you don't have to worry about crowds. The trail is an old Qing dynasty road which used to be the main route into northern Yilan county. The trail runs from the town of Dali 大理 in Yilan county, through the mountains, and down to a riverside park a few kilometers above Fulong 福隆 beach in Taipei county. The two towns are connected by railroad, so it's very convenient to make a circuit. It is also very accessible from Taipei by train, and makes an excellent day trip for anyone who wants to get out of the city and enjoy nature.
There is just one problem with this trail - the trailhead on the Fulong side is several kilometers from the train station, and there isn't any reliable public transportation. So if you come out on this side, you have to walk down the mountain beside a busy road. For this reason, you should definitely start on the Fulong side and finish up at Dali.
I suggest you start the trip by taking the train to Fulong. This is easy to reach from either Taipei or Yilan. If you want, you can go to the beach at Fulong, or else rent bicycles and explore the area. There are several kilometers of cycling paths around the town.
When you are ready to start walking, you can get a taxi in front of the train station and go to the trailhead. This is in a pleasant riverside park. If you like, you can go down to the river and go for a swim. The cool water feels wonderful on a hot day, and some stretches of the park are really beautiful.
When you're ready, just start walking up the trail. The path is made from large flat stones and is well maintained. At first you will pass through some fields with water buffalo. Then a humid rainforest. The stones get slippery here, so be careful. The trail slopes gently uphill for the first 2/3 of the route. It's not very difficult. You will pass two old inscribed stone tablets, which are major tourist attractions.
There are lots of bilingual signs along the way explaining historical facts and introducing the flora and fauna, and maps along the way showing how far along the route you have come.
Eventually the forest thins out and you are walking through a grassy mountain landscape. During the descent down into Yilan, there are spectacular views of the ocean and Lanyang plain.
At the Dali end of the trail there is a visitor's center complete with coffee shop, so you can sit down in the air conditioning and relax with a cappucino if you feel like a taste of civilization. From here you can walk down to the main road, turn right, and walk a hundred meters or so down to the train station to get home.