Leviticus 9:15

‘And he presented the people's oblation, and took the goat of the purification for sin offering which was for the people, and slew it, and offered it for sin, as the first.'

Now he was in a position to offer the people's oblation (gift which they were obliged to make). The elders mentioned in Leviticus 9:1 may have provided representatives for the slaying of the beast, or it may be that Aaron himself slew it as the people's representative (depending on how literally we take ‘he slew it'), and Aaron then offered the purification for sin offering in accordance with the required method, as he had done with his own purification for sin offering, which had also been on behalf of the people because he was their representative. He would be ultra-careful, at this his first attempt, to ensure that the whole procedure was correctly carried through. He must have been as nervous as any novice.