Doodh sagar means 'a sea of milk' which is exactly how the waterfalls is. It is located on Karnataka-Goa border and became popular after it appeared in the film Chennai Express. Until recently, trekking to this waterfalls was banned but now the ban has been lifted.
There are several options of trekking to Doodhsagar waterfalls. For travellers from Karnataka, especially Hubli/Dharwad/Belgaum, the best option would be to - travel by train to Castle Rock; trek for 3-4 hours from there along the railway tracks.
Note: Make sure you reach Castle Rock in the early hours of morning so that you can trek back on the same day.