Fast travel to Ridgeline Base Camp in Geothermal Valley, which can be accessed after progressing through most of the story to reach this area. The base camp is located at the top of a waterfall, and just off to the side you can see a wooden platform with a red mark on it 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 perform a swan dive by pressing LT. If done with the correct timing, Lara will enter the swan dive downwards and you will get the "No Guts, No Glory" achievement upon hitting the water. If you fail, climb out of the water, ascend the hill, and try again.