The workers who did this at Jeroboam's commands are represented by the metonym of the name of Jeroboam himself. AT: "Jeroboam's workers made houses on high places" (See: rc://en/ta/man/translate/figs-metonymy)
The implied information is that these were houses of worship. The full meaning of this statement can be made explicit. AT: "houses of worship on high places" (See: rc://en/ta/man/translate/figs-explicit)
"appointed men to be priests"
This is the eighth month of the Hebrew calendar. The fifteenth day is near the beginning of November on Western calendars. AT: "on the fifteenth day of the eighth month" (See: rc://en/ta/man/translate/translate-hebrewmonths and rc://en/ta/man/translate/translate-ordinal)
"Went up" here is an idiom for going to a sacred place to worship since these altars were located on high places. AT: "offered sacrifices on the altar" (See: rc://en/ta/man/translate/figs-idiom)