Does the prohibition of offering sacrifices outside of Jerusalem, apply to gentiles? If not, how would they find out how to construct the alter, etc.?

Non-Jews are allowed to sacrifice wherever they wish, and Jews can direct them how to do so, see Zevachim 116b, where Rava instructed gentiles in Babylon in detail, how to sacrifice for Shvur Malka, king of Persia, several hundred years after the destruction of the Temple in Yerushalayim.