Depends how much time you have

If no time you can fly from dempaser to Medan and then you used to be able to fly from Medan to Nias
Alternativly Bus to Jakarta, Bus to Medan Bus to Sibolga boat to Nias - allow a few days/nights
Or do a bus to West Coast Java visit a few places along the south coast boat over to Sumatera, check a few places out wind your way to Sibolga and get the boat over


If you choose to overland be prepared for delays, difficult people busses not turning up, people harassing you and ardous travelling though from what i am led to believe bussing around Sumatera is much much easier than it once was.

What i did in the past was fly into Medan(from Perth) coast my way casually to Nias then once bored there overlanded slowly to Bali, chilled there then flew out from Dempasar.

If your dead set on Bali being your destination there are probably operators in Kuta who offer packages - but you are missing out on so much doing it this way

anyone with more detailed info?