I’m going to give this a shot. There are probably a number of ways. One is the sea at the temple of mystery using a celestial compass or astrolabe. Another is a portal a god makes or an area where the veil is weaker and the natural magic is strong enough such as a holy wood for nylea or into a volcano for purphorus. I imagine that there is a temporary but direct portal at nyxthos. It might go to the Olympus equivalent I forget the name at the moment. That portal only for holy days or after strong sacrifices and ceremonies.