For this achievement, as soon as you land in Xen (which you can chapter select to) your goal is to jump to the island with the first long jump ramp on it without going through the caves. The opening Xen island refers to the island you start on NOT the second one.
When you walk toward the big set piece with all the flying aliens, instead of going left turn right. You need to long jump around the edge of the rock wall and land on a large floating rock, then long jump further right to the lower platform. If you land and there's a long jump ramp to your immediate left you're in the right place. Don't jump to the water area after that, it won't trigger there.
Achievement pops as you walk to the ramp.
This video at this timestamp shows exactly what you need (not mine).
You can jump to the smaller rock but don't have to. Gordon has enough air control that you can just curve around to the larger one. Do that if you find yourself accidentally bouncing off things into the void.