‘And the sons of Aaron presented the blood to him, and he dipped his finger in the blood, and put it on the horns of the altar, and poured out the blood at the base of the altar, but the fat, and the kidneys, and the covering from the liver of the purification for sin offering, he burnt on the altar, as Yahweh commanded Moses.'
Then he switched roles, for he moved to the altar ready to receive the necessary parts of the sacrifice from his sons who had meanwhile been acting as priests. His sons, who had caught the blood in a vessel when Aaron slew the calf, then brought the blood to Aaron, who dipped his finger in the blood and applied it to the horns of the altar to purify the altar which would offer up his purification for sin offering.
Normally in the case of a purification for sin offering for a priest the blood was to be taken within the Holy Place and sprinkled before the veil, and be applied to the horns on the altar of incense (Leviticus 4:6-7). But as yet no one apart from Moses had entered the Holy Place, and he had not defiled it by specific sin. Therefore no purification was needed there. All that had had contact with Aaron was in the courtyard, and the most holy of these was the altar. The blood was therefore applied to the horns of the altar.
Then the blood was applied to the base, and the fat and vital parts were burned on the altar, just as Yahweh had commanded Moses. This is the last reference to Moses' obedience to God's commands, an idea which has been repeated again and again in order to emphasise his obedience (see Leviticus 8:4-5; Leviticus 8:9; Leviticus 8:13; Leviticus 8:17; Leviticus 8:21; Leviticus 8:29; Leviticus 8:34; Leviticus 8:36; Leviticus 9:6-7; Leviticus 9:10). All that was done was done at God's command, and Moses obeyed implicitly and without question. But from now on obedience is in the hand of Aaron and his sons. Moses was no longer involved. Sadly it would not last for long, and a terrible lesson would have to be learned, that God's ordinances must not be interfered with.
The fact that the phrase ceases here demonstrates that it was the writer's intention to bring home the obedience of Moses. Had it just been a stereotyped phrase it would have continued. He wants us to know that Moses had done in all this exactly as God had commanded.