Face the crates and the fence. There are two movable crates on the ground. The left one differs from the others. Leave it for now and push the right one (1) to the end towards the fence, and then push it to the left against the stacked crates.

Climb on the crate (1) you just pushed and push the crate (2) that is now in front you to the end.

Drop to the ground. Now you have to move the first crate (1) to the front side of the stuck. So, facing the fence, pull it twice and then go to the right side and push it to the left.

(Note - Some times, after pulling this crate, you cannot push it to the side, so you need to move it circularly. Pull it three times, then go to the right side of it and push it once. Then push it towards the stacked crates.)

Now go to the crate (3) that differs from the others and push it towards the stacked crates and next to the first crate.

Go to the right side of the stack (assuming you see the fence) and climb on the box that is closest to the fence. Turn so that you have the fence to the back of you and push the crate (4) on the top of the first crate (1).

Now, stand on the different crate (3) and pull the crate you just moved (4) as much as you can (press Action and the down direction key many times).

This will result half of the crate (4) to be on the different one and the other half on the first one. Go to the other side and push it on the top of the different crate (3).

Climb on the crate (4) you last moved and push the topmost crate towards the fence. When you do that, Lara gains a strength upgrade.