The internal routing for the dropper post is a tube that runs from the entry port on the drive side down tube right at the head tube and exits at the bottom of the seat tube. You can shine a light in the seat tube and see it sticking up. All you need to do is push the housing thru that tube and it'll come out the seat tube. I prefer to NOT have a cap on the housing when pushing it thru the tube, just to prevent any chance of it getting pulled off inside the tube.