Exodus 35 - 40

Throughout Exodus, the Lord gives specific instructions of those who are to administer -

"...In the tabernacle of the congregation without the vail, which is before the testimony, Aaron and his sons shall order it from evening to morning before the LORD: it shall be a statute for ever unto their generations on the behalf of the children of Israel." (Old Testament Exodus 27:21)

The Lord gives instructions on how they were to perform sacrifices and how they were to dress. But the most important instruction from the Lord to Moses with -

"...thou shalt bring Aaron and his sons unto the door of the tabernacle of the congregation, and wash them with water.
13 And thou shalt put upon Aaron the holy garments, and anoint him, and sanctify him..." (Old Testament Exodus 40:12 - 13)

Merriam-Webster Dictionary defines "anoint" as -

"...to apply oil to as a sacred rite especially for consecration..."

Sanctify is defined by the same as -

"...to set apart to a sacred purpose or to religious use - consecrate; to free from sin - purify; to impart or impute sacredness, inviolability, or respect to..."

All this for the purpose -

"...that he may minister unto me in the priest's office..." (Old Testament Exodus 40:13)

And to Aaron's sons -

"...thou shalt anoint them, as thou didst anoint their father, that they may minister unto me in the priest's office: for their anointing shall surely be an everlasting priesthood throughout their generations." (Old Testament Exodus 40:15)

It becomes apparent that those who minister in sacred things do not do so of their volition, for -

"...no man taketh this honour unto himself, but he that is called of God, as was Aaron." (New Testament Hebrews 5:4)

And those called should be washed clean, anointed, and sanctified to minister in such sacred things for when -

"...a cloud covered the tent of the congregation, and the glory of the LORD filled the tabernacle." (Old Testament Exodus 40:34)

For the Lord has declared in modern day -

"I will appear unto my servants, and speak unto them with mine own voice, if my people will keep my commandments, and do not pollute this holy house." (Doctrine and Covenants Section 110:8)