You’ll probably need an OS map for this; it is fairly scenic, and straightforward ride, starting on road, then forestry tracks, but away from everything.
Take the small roads (Old Military Road / B796) up behind Gatehouse-of-Fleet towards the old railway line; follow that or the road East to Water of Fleet viaduct, crossing at the ford below. Follow forestry roads North past Loch Skerrow, to the A712 SW of Clatteringshaws loch.
From there you can do a loop coming back to the A712. Go round Clatteringshaws to the N end, E route is more attractive, W is shorter; take the Old Edinburgh Road North of Clatteringshaws, then loop round on paths to the A712 just E of Murrays Monument and Grey Mare’s Tail waterfall on your right.
The latter sections are quite pretty.
Pretty close to Kirroughtree here.