Verses 3-8:
Moses descended from the Mount and relayed to the people all the laws God had given, recorded in the last three chapters. All the people readily pledged to do all God had commanded. Subsequent events show that they were unwilling or unable to keep their pledge.
Moses then wrote the Lord’s words in a book. Early the next morning he built an altar upon which to make the sacrifice that would ratify the covenant Israel had made with Jehovah, to obey all He had commanded.
"Young men" were likely the firstborn sons, who were next in line for the priesthood in their families. This was prior to the establishment of the Aaronic priesthood.
The blood of the sacrificial animals symbolized their life, and was an essential part of the ’sacrifice. It was usually poured or sprinkled upon the altar. In this instance, Moses put half of the blood in basins, to be sprinkled upon the people. This signified the unity of the people in their solemn pledge to the Covenant (cf. Heb 9:18-20). Following the usual custom, the other half was then sprinkled upon the altar.
Once more Moses reported to Israel the legislation Jehovah had given on the Mount, this time reading it from the "Book of the Covenant" which he had written, under the direction of the Holy Spirit, 2Pe 1:21. And once more the people solemnly promised to obey all God had commanded.
Moses ratified the Covenant by sprinkling the blood of the sacrifice "on the people." It is unlikely that this included each individual Israelite, as they numbered about two million people. It is more likely that the "elders" were sprinkled as symbolic of the entire nation.
The sprinkling of the blood on both the altar and the people denoted the unity of the people with the altar, in the ratification of the Covenant.