No need to remove your seat post!  Just remove the two pre-installed stainless screws using the included 2mm hex wrench, open the mount and wrap it around the seat post.  You might have to give it a bit of squeeze to line up the screws with the holes - they go in at a 45 degree angle which provides excellent clamping strength.  Tighten the mount until the gap is closed and then tighten some more to ensure the mount doesn't slide on the post. Don't be shy about making both screws tight enough to completely close the gap on the mount like the video below shows. Try not to use a ball-end hex wrench as they don't work well with the small screws. Installation takes about a minute.