I had a little trouble finding Hemenway Road and the trailhead, then was a little confused on Hemenway Road when I saw another, longer trail that intersected with it. But I figured it out and started from the trailhead and parking area at the corner of Hemenway Road and Great Hill Road. The gravel lot can accommodate four cars or so. It is part of the Hemenway State Forest. The map on AllTrails of the site shows the route I took more clearly than the Peakery one:
https://www.alltrails.com/explore/trail/us/new-hampshire/great-hill-fire-tower?mobileMap=false&ref=sidebar-static-map
As the trail ascends Great Hill Road east of Hemenway Road, it is an old jeep trail type road, and is in very good shape. The trail, though, is not consistently blazed. Going up the south loop, as the AllTrails map shows, follow the most used/improved trail at all intersections. There is a left turn about halfway in or a bit more that's the most key example. There are snowmobile trail arrows pointing left there, and the road is more improved; go that way. There is a retired fire tower at the summit which offers very good views, particularly to the east.
Going down, one can make a partial loop (which eventually rejoins Great Hill Road)by turning right and taking the north loop. It is not nearly as much of a road as the south loop, and it takes longer. But it features several apparently abandoned wooden buildings that add interest to the hike. The one closest to the summit may have been the warden's cabin. Further down, there is a storage building, and finally, a lean-to which could have accommodated two sleepers, but now has a softball-sized hole in the roof. The first two are boarded up. One has to turn left twice on this loop, returning eventually to Great Hill Road shortly after crossing a metal gate.
Great reward for a a relatively short hike! I'm glad I found it.