This is a short trail along a ridgeline with great views, beginning from Phu Chi Dao, which is in between Doi Phatang and Phu Chi Fa but is quieter and slightly higher than both.
Google map 'Phu Chi Dao Entrance' for the turning off the main road, because the signs are only in Thai. Once off the main road for about 100 metres you have to take a left and from there it is a dirt road with no other buildings or turnings until you get to the carpark from which point you can start walking. By the way this road is only passable by 4x4 or off road bike, or scooter if you have A+ scootering skills. After rain or in rainy season it's probably not possible with a scooter. I did this end of the rainy season 2020 and the road was in pretty bad shape. The locals will take you up by 4x4 for a fee, or you could walk but it is a couple of km and all uphill; feels like more.
From the carpark to the peak is partly by manmade steps until you get to the ridge and from there it's plain dirt. This is the top:
I went here on consecutive days because everything was blanketed in fog the first time. The next day was clear. Lots of insects around...
When you come back down from the top instead of going back in the direction of the carpark, stay on the ridgeline and follow a less-travelled trail where these pieces of wood have aspirations of becoming a fence:
By now looking back whence you came you should be getting a view something like this:
You can see Phu Chi Fa Peak on the horizon, just to the right of Phu Chi Dao, it's the peak with a sheer cliff which looks like a beak or a shark fin.
Carrying on walking you will enter forest for a while where the path is not so obvious, but since this is a ridgeline walk it is not difficult to stay more or less on course. There are some signs of camping around and I found wild persimmon fruit. The seeds are a bit thinner than from the persimmons you buy at the market.
I also saw this caterpillar which I think is covered in parasitic wasp cocoons. I read the larvae form inside the caterpillar and then burrow out and make their cocoon, and apparently the caterpillar does not always die. This one was still alive and seemed healthy enough.
Once out of the forest and approaching Phu Chi Duean (ภูชี้เดือน) you should get views of what I thought was Doi Phatang, but actually upon closer inspection of the map I think it is a peak only accessible from the Laos side, which is not labelled. Doi Phatang is behind it, not visible.
I stopped at Phu Chi Duean, but the trail did seem to continue onto that Laos Peak, I wondered if it would continue all the way onto Doi Phatang but I can't see any trails on satellite view going that far.