Jeroboam thought to himself, The kingdom is in danger of reverting to the house of David.
If these people continue to sacrifice at the LORD's temple in Jerusalem, they will again become loyal to their master Rehoboam, Judah's king, and they will kill me so they can return to Judah's King Rehoboam.
So the king asked for advice and then made two gold calves. He said to the people, "It's too far for you to go all the way up to Jerusalem. Look, Israel! Here are your gods who brought you out from the land of Egypt."
He put one calf in Bethel, and the other he placed in Dan.
This act was sinful. The people went to worship before the one calf at Bethel and before the other one as far as Dan.
References for 1 Kings 12:30
Jeroboam made shrines on the high places and appointed priests from all sorts of people, but none were Levites.
Jeroboam set a date for a celebration on the fifteenth day of the eighth month. It was just like the celebration in Judah. He sacrificed on the altar. At Bethel he sacrificed to the calves he had made. There also he installed the priests for the shrines he had made.
References for 1 Kings 12:32
On the fifteenth day of the eighth month—the time he alone had decided—Jeroboam went up to the altar he had built in Bethel. He made a celebration for the Israelites and offered sacrifices on the altar by burning them up.
References for 1 Kings 12:33