This can be attained in Geothermal Valley
closest to the Ridgeline
Base Camp which can be accessed after progressing to this area during the story. This base camp is located at the top of a waterfall and just off to the side you'll notice a wooden platform that pokes out of the side of the cliff edge.
Get a running start (
) towards the platform, jump (
) once you reach the end, and quickly enter the swan dive by pressing
. If done with the correct timing, Lara will enter the swan dive downwards and unlock the achievement upon hitting the water.
If you fail, climb out of the water, ascend the hill and try again.