Reet!
To get to Slackdhu, you can either ride then push up from Cantywheerys or you can ride up the red Land Rover track off of the pipe track just before you get to Killearn. Take the right fork and there is a quad path all the way up onto the Campsies. There are three steep rises after the first big climb. At the cairn, double back for about twenty or thirty yards and you will find an initially indistinct trail that goes in a circuitous route towards Slack Dhu. There is a fair bit of tussock bashing (mostly rideable in the summer or best on a frosty day) until you hit the edge of Slack Dhu. You can follow the edge of the cliffs and eventually drop into Blanefield behind the Catholic Church. Or just ride the steep descent to Cantywheerys. It flows well with some lovely sections of steepness.
On the big climb, you can clean it all with the right gears and have a go attitude. It needs finesse and pacing though! One by would not be your friend!
You can also follow quad bike tracks just below Earls Seat all the way to Fintry via the two big Corries. There is some hike a bike while there is also 5 to ten minutes of mostly rideable trailless hillside to get to the old Roman Fort. The descent off it is steep!
The descent off Dumgoyne where you traverse the hillside before dropping down is great. Don’t try the main path up. There is one to the left (looking up the hill) that is less death on a stick although still tops out at 47 degrees for some way. I love it! You can also do the last part as a plummet from the trail up towards Earls Seat.