Found this on trip advisor. Wanted to do it but didn't want to pay $75 to do it on a tour. It's perfectly possible alone. I found another guy from my hostel who wanted to go (the more, the cheaper).
Took the bus from Paraty bus station to Paraty Mirim (R$3.50) then haggled with a speedboat driver to take us round the bay to Saco do Mamangua. We found a couple who wanted to do it 2. It cost us R$180 for the boat round, 2 hours on the walk and then the boat back. The guy waited while we walked. Others could probably haggle a better price but that's what we managed. So in total, it cost me personally R$52. Much better than $75 Usd.
The walk itself is easy to navigate. Follow the track on the left side of the beach if looking from the water. There is 1 fork in the path, take the right route that goes up.
Take care coming down as it's slippery this time of year. The view is incredible and well worth it. We took a packed lunch and ate it at the top. More