Use the Ectoplasmic Distiller near incorporeal undead to collect 12 Scorched Ectoplasms in Silithus, 12 Frozen Ectoplasms in Winterspring and 12 Stable Ectoplasms in the Eastern Plaguelands. Bring them along with the Ectoplasmic Distiller back to Mux Manascrambler in Gadgetzan.
A fine piece of engineering, if I do say so myself! You will need to travel a bit, I'm afraid, in order to find the incorporeal undead we require.
You see, different climates lend different consistencies to the spectral fabric of the dead. The tortured night elves of Silithus, the spirits of the highborne from Winterspring and the ghosts and banshees of the Eastern Plaguelands should give us the different types of ectoplasm we need. Be sure to place the distiller nearby before you harvest them!