Your target is the middle root of the tree and specifically the outer corner that looks at your side. This jump does not require a special set up, as long as you have a good run up (at least two hops back from the corner). If you try to set up the jump by positioning Lara in the corner close to the wall, it is more possible to bang on the corner of the wall and fall into the water. If, however, you need a set up for the jump, here it is:

Walk up to the corner, but do not stick to the wall. If you do that, Lara will hit on the wall whilst running or you will have to press the right direction key to prevent her from hitting on the wall. To stand as the left hand picture shows, walk back a bit and whilst pressing Walk and the direction keys, position her on the correct spot.

Turn left and aim the outer corner of the middle root. Tap back at least twice and start running. When Lara reaches the corner, jump.

Lara will jump towards the middle root and whilst in the air, you may notice that she is not heading to the corner. In any case, do not let the up direction key before Lara steps on even ground. Once she lands on the root, release the up direction key or you may proceed further and fall into the water.