After you collect the items of the 3rd Secret in the waterfall area behind the swamp, climb on the wall that separates the waterfalls from the swamp. Supposing you have the swamp in front of you, go left. Run to the other end of the ledge and then go right to step on the greenery. Stick to the left side, because on the right side the leaves are not solid and Lara may fall through them and to the ground (where the yellow X shows).

You can see a green slope before you now, with a slanted top. Move towards the slope. If you turn a bit to the left, you will be able to see a slope behind (where the yellow dot shows). This is where you need to go and the jump is quite tricky. The screenshot below shows how you need to stand in order to perform the jump, and the text describes the way.

Move towards the slope. When you are close to it, turn right. Walk to the edge of the greenery overlooking the swamp (the waterfalls are to your right, the slope is to your left). When you can go no further, turn to face the slope again and walk up to it (it is not necessary that you do this, but it will help you get the correct position for your next movement).

Now, take a few steps back (about six steps) and Lara will stop where the leaves are not solid anymore. In case you are not absolutely at the edge, side-step a bit to the right. Now, you have opposite you the edge of the slope. Angle Lara a tad to the right and start running towards the edge of the slope – but because you have her angled a little to the right, she will not bang onto it, but pass slightly to its right. As soon as she does so, immediately press the left direction arrow while she is in the air. Lara will face upwards, land on a sloped ledge behind the green slope and slide backwards onto it.

Press Action and she will grab the edge of this ledge. Shimmy to the right (if Lara refuses to do it, release Action and immediately press it again, or press the up direction arrow to make her pull up the ledge and slide down grabbing its edge again) and pull up to an even green ledge (where the yellow dot shows).

This ledge is made of leaves, that are seemingly sparse, but in reality there is a solid invisible ledge under them. So follow the green ledge as it goes to the left and when you are close to the greenery wall turn right and walk carefully on the invisible ledge. Lara will stop when she cannot go any further. Jump over to the green ledge ahead. A simple jump should be enough if you are standing at the edge.

Walk carefully along this long green ledge. You will reach a point where Lara's head will stick out of the leaves above and she will not be able to proceed. So, press Duck and crawl forward. When you reach the wall, Lara will be able to stand. Turn right towards the swamp. Keep close to the wall and walk forward. When Lara stops, turn around and jump backwards pressing Action to grab the edge of the greenery. Safety drop on the steps.