Finding the Map
The largest hurdle in finding the High Stakes treasure is finding the map itself. You'll need to witness a random encounter somewhere along the heartlands anytime after Chapter 2 begins. You'll spot a white dot on the radar and will be able to hear a man mumbling to himself about the treasure, usually overlooking a cliff somewhere. There's no easy way to go about this so you'll have to look at the map above and check out each of the locations until you come across him.
Once you find him, head over to him and before he can react, use your Lasso to bound him and then hogtime him, you'll then be able to head over and pick up the treasure map. If he somehow escapes then you'll get two more shots at robbing him in another location, fail that and you'll have to resort to checking out a Fence to buy the map from.
You'll notice right away the waterfall in the new map you've just acquired, this is in fact the Cumberland Falls found to the east of Wallace Station. Make your way to the waterfall and then head over to the east side of it, following the path until you can head down towards the riverside. Make your way along the side here towards the waterfall and travel across the log to reach the waterfall itself. Head inside the waterfall and check the back of the wall to find the next treasure map.
The first detail you'll notice on this new map is that of a frozen lake, this is Barrow Lagoon found to the north, in Ambarino. Mount your horse and make your way over to the frozen lake where you'll notice a higher island found in the northeast of the lake. Take your horse around to the north side and leap onto the rock with a log extending to the island. As you cross the log, you'll notice a hole in it, search this to find the final map.
The Treasure itself can be found in a rather elusive place that you'll need to be careful with your jumps to discover. Make your way over to Fort Wallace until you're outside the front gate, from here take the eastern path and then follow it to the mountain there where you'll find a small path leading down to the cliffs.
Take a left here and head up the thin path until you reach the end, look down to see a rock that you can leap to. Once you land, look behind you and down to spot the next rock to leap to before finally jumping down to the final rock. From here, follow the path under the hanging rock and at the end of this, you'll be able to search this rock for the treasure.