For this achievement you need to stick 3 water treatment rooms in a row. The water rooms cost 175 each so it will cost you 525 caps in total. You can ether make the rooms when you first spawn into your shelter ( WARNING this will ruin your game and you will have to restart) or you can wait till about 10-15 mins into the game and you will have enough caps by just playing the game normally.
If you would like a video guide for this though I will leave mine below