Following Elm Tutorial (https://www.youtube.com/watch?v=6PDvHveBtDQ)

tutorial.elm

import Graphics.Element exposing (..)
import Graphics.Collage exposing (..)
import Color exposing (..)
import Mouse
import Window

relativeMouse : (Int, Int) -> (Int, Int) -> (Int, Int)
relativeMouse (origin_x, origin_y) (x, y) = 
  (x-origin_x, origin_y - y)
  
center : (Int, Int) -> (Int, Int)  
center (w,h) =
  (w//2, h//2)
  
render (x,y) = 
  let forms = [circle 15 |> filled lightBlue
                         |> move (toFloat x, toFloat y)]
  in
  color gray <| collage 400 400 forms
  
main = Signal.map render <| Signal.map (relativeMouse (200,200)) Mouse.position

Part 2: https://gist.github.com/fredguth/aa87dc1cba583a78738f

Part 3: https://gist.github.com/fredguth/72f8c8b8ae387c4c154b

Worth to mention that this code is a more updated version of the code in this playlist.