Route from Bangalore: SH17 to Mysore, NH 212 to Gundlupet; state highway to Coonoor via Bandipur National Park, Gudalur, Ooty, Aravankadu, and Wellington. There are many signboards along the way from Mysore directing you towards Ooty. Once in Ooty drive through the city, there are boards along the way showing you the way. Roads are very good from Bangalore uptil Bandipur, but very sad later on.
From elsewhere in India: Fly to Coimbatore or come by train. You can drive down to Mettupalayam about 1 hour's distancefor the toy train or hire a car to drive up to the hills. Coonoor is roughly about two hours drive from Coimbatore and the views are breathtaking as the road takes 14 tortuous bends upwards through dense forest.
If you are coming from within Tamil Nadu, you can take the train to Mettupalayam and then the cute little toy train up to Coonoor and Ooty. There are regular buses coming here as well.