Through ceremony. Public ritual predicated around huacas was used to explain heritage, mythology, etc. It was also applied retroactively (kind of cheating the concept of time to claim rights to a land). Harlan Coben's brother, Lawrence Coben, (archaeologist/anthropologist brother) that has a solid chapter on it. It's in Archaeology of Performance. So, to answer the question, oral tradition (duh). But more importantly, and what I think you're indirectly asking, the motivation to do such was, again, huacas/religion. This is especially true of the creation myth, with the caves etc. I can't remember which, but a very well-known Inca site has window slits symbolizing the caves from the creation myth. Someone from BYU, Zachary Chase, has a good chapter in another book that's along the similar vein. To answer the khipu part: no. We also have several chronicles written from Spanish speaking inca (without getting into the class system) that detail mythology a bit, but some are more white washed in favor of Christianity than others. There are some examples of tradition being passed down on a regional basis (see: staple financing theory/public monument construction, which continues to this day on the same material by some local villages, like rope bridges). I can't help with the time line, though. I'm not an expert, I just read a shit ton of lit reviews for a class I took before I graduated.