Building the Second Temple's altar - at which point, they already knew the outline of the building itself, but they weren't quite sure about the altar. How did the know? One suggestion is that the archangel Michael told them in a vision. Alternatively, the "ashes of Isaac" -- as seen in a vision -- were seen in the same place as the altar, namely, since that was also the location of the Binding of Isaac. Alternatively, the scent at the place was that of the offerings, instead of the scent of incense that was designed to improve the overall smell of the place. Plus 3 prophets revealed the necessary details of replacing the altar -- and maybe also to use the now-classic Torah scribal script for writing a Torah scroll. Also, the ramp to the altar were separated by a gap -- which means that the offering was thrown across the gap, and the practice of doing so was derived from a verse.