Moses poured some of the anointing oil on Aaron’s head and anointed him to consecrate him

 Leviticus 8:1-21

Then Moses took the anointing oil and anointed the tabernacle and everything in it, and so consecrated them. He sprinkled some of the oil on the altar seven times, anointing the altar and all its utensils and the basin with its stand, to consecrate them. He poured some of the anointing oil on Aaron’s head and anointed him to consecrate him. Then he brought Aaron’s sons forward, put tunics on them, tied sashes around them and fastened caps on them, as the Lord commanded Moses.

He then presented the bull for the sin offering, and Aaron and his sons laid their hands on its head. 

He then presented the ram for the burnt offering, and Aaron and his sons laid their hands on its head. 

Now that the priest's sacrificial duties have been listed. Moses implements the instructions given in Exodus 29. In an elaborate and impressive ritual, Aaron and his sons are instituted to the priesthood. Moese performs the priestly duties on their behalf. The blood on Aaron's ear, hand, and toe indicates the dedication of the whole man to God's service, to hear and carry out God's instructions.

Why?

Anointed the tabernacle and everything in it, it is clean everything in the tabernacle be holy and clean.

Moses poured some of the anointing oil on Aaron's head and anointed him to consecrate him. This is a mission from the Lord to Aaron, and his sons to serve the Lord.

To serve the Lord who is chosen one.

How?

Jesus Christ be our sin offering so we may be sons of the Lord, His blood has anointing over us through the Holy Spirit, this is the great grace to all Christians.