v1-8: The law had been laid down and explained. Moses had told the people what God had said, and now it was time for decision. The children of Israel had the choice of whether to come into Covenant with God.
v1: God's instruction, "Come up to the Lord". He graciously invites people to meet with Him.
v2: Aaron, Nadab and Abihu, with the seventy elders, had to stay at a distance. But Moses was allowed to draw nearer to God. He continued to enjoy a special closeness to God.
v3: Moses reported all the Lord's words, as recorded in ch 20-23, to the people. The people agreed to obey all that the Lord had said, as in 19.8. The danger was that, in the thrill of all that they had seen and heard, they made a decision too hastily.
What would have happened if they had said that the law was a burden too heavy to bear? (see Acts 15.10); we need Jesus Christ to fulfil God's righteousness for us.
v4: So that the people could be reminded of the Lord's words, Moses wrote them all down.
v4-6: Moses prepared a sacrifice, indicating the entering into the Covenant by the people. Offerings of worship, burnt offerings and fellowship (or peace) offerings were made.
v7: Again the people promised to obey all that the Lord had said.
v8: The Covenant was sealed with blood, see Heb 9.19-22, and the children of Israel were now "under law", debtors to keep the whole law (Deut 27.26; Gal 5.3). They would therefore receive its blessings AND its curses. It their relationship with God, this was a crucial milestone for the Israelites.
v9-11: From v1, Moses and Aaron, Nadab and Abihu, saw the Lord God of Israel. Here they draw nearer than before, and even ate and drank in His presence. Yet, within a short while, Nadab and Abihu were negligent and careless in their approach of God (Lev 10.1-2).
v12: The Lord God wrote certain words on tablets of stone. These contained simply the Ten Commandments, as in 20.1-17. Moses alone could come onto the mountain, and stay there.
v14: In Moses' absence, Aaron and Hur, with the, presumably seventy, elders, were left in charge.
v16: Moses went up onto the mountain, Mount Sinai. He had to wait six days before meeting with God.
v17: From outside, the cloud looked like a consuming fire, but inside, Moses was totally safe.
v18: Moses remained forty days with the Lord. Elsewhere this time indicates a period of testing, see Gen 7.12; Matt 4.2. The testing here was for the Israelites.
The Lord had given instructions about practical obedience (ch 19-24); now He explains details of worship (ch 25-31).