The ritual does have some constants, at least. Water as a vector, the presence of an arcana symbol. It seems to happen at a pre-determined time of the month.

[But even that is barely anything to go on, and he knows as much. He might be better off simply finding his own way, perhaps using the existing ritual as a guide to invent something entirely new.

Which is why he's interested in hearing about the ways others, from different places, might travel across other worlds. Ciri goes there before he has a chance to ask, and it's easy to see him perk with interest, even if the people in question are likely less-than-friendly.]

How do they do it?