Along Holland Road, near a road called Taman Nakhoda.
Beside a construction site, you can see a container shack with a very low metal bar across a road leading into the forests. Take that route. Walk straight.
Until you see this:
Turn right, walk thru the plants (very low, don't worry).
Walk across the grounds, straight. You would see a curved path up a small hill.
Follow that and you'll reach Istana Tyersall!
OR
From Tyersall Avenue, the Istana's gates. Walk along the road, till you see this:
Turn left. Follow the path up the hill and you'll reach it!