First the earth cooled. Then the dinosaurs came, made sequels about eating small children, and died of shame from bad reviews and overblown special effects. Then a group of old actors, actresses, directors, and producers decided that they should give themselves awards every year for being beautiful and perfect. The answer is members of the Academy. Suffice it to say that they are members of the industry.
Every member belongs to one and ONLY one branch.
Music branch members nominate people for song and score, while actors only nominate actors. For these categories, a pre-selected panel picks the nominees. Then a very reputable accounting firm with damn sexy accountants tabulates the votes. In mid-February, the top 5 nominees are announced in each category, and at the end of March at an awards ceremony, each winner is announced. Well, if you're gonna predict the Oscars, it helps to know who is doing the voting and how people got nominated in the first place.