With material like the Cauldron of Poesy text, we have some statements about cauldrons in the body and what they do, and we can infer about where they were situated. But when it comes to actual operations and getting those cauldrons to do what they're supposed to, the only thing we can really do is look at comparable systems from other cultures and do some experimentation. Will what I come up with be exactly what the pre-Christian practitioners of the art were doing? No. Will it have the same or similar results? Well, if it results in imbas as an upwelling of poetic inspiration and magical and spiritual power or enlightenment, then yes, I think the results are similar to what was intended. And if it works then this is the direction in which we should be moving.
Within CR there are a lot of branches that people find of interest. Within any given culture you're going to find householder paths, warrior paths, magical paths, healing paths, storytelling paths, and more. And each of the various Celtic cultures adds another layer onto that complexity. You're much more in luck if, for instance, your interest lies in Irish material about household traditions than if it lies in the deities and practices of Gaul. It will be much harder to understand and reconstruct Gaulish practices because of the lack of easily available material. There is no extant Gaulish literature that describes deities and myths. Most of what is known is from archaeological sources, with a few inscriptions on altars or on spell tablets. A little more is added from references to Gaulish people by Greek or Roman writings.
A lot of Irish household tradition has survived into modern folk practice, though very much changed and buried within Christianity. It is by no means a fully realized polytheistic Pagan practice at this point, but it's a good and necessary basis to grow from. For Irish Pagan mystical and spiritual practices the material is rather less plentiful and so more has to be reconstructed or created by reference to other things. It's regrettable, but it's how things are.
So what's a person without a lot of sources to do? Here's where our uncertainties lie, and this is where a lot of people get discouraged. But comparative work helps in many cases. Nearby cultures can be examined, of course, and the field of Indo-European studies has a lot of useful material. Offerings are a practice common to every culture I've ever heard of, so that's always a good place to start -- and the archaeological record often provides fairly good material on what kinds of objects and foods were offered by any individual culture or people in a given area.
But ultimately the words of our practice will always need to be our own. We can base them on material in sources like the Carmina Gadelica, but that text, wonderful as it is, doesn't cover all contingencies or address every circumstance for which we might need a ritual text. And if filidecht is about the creation of poetry, then always using someone else's words is a violation of that tradition.
We know that the tarbhfeis ritual of Ireland involved a fili who was surrounded by four people chanting truth spells, but we have no idea what the words of those spells were. We don't know if the same one was used each time, or if they were extemporaneous compositions created by each of the chanters. Imbas is what helps us to create appropriate words for situations like these and experimentation can tell us if those words work or not. We can't know until we try it.
Practice is about what works. We can theorize all we want, but until we commit something to physical ritual, until we commit words to speech, theory is all that we'll have. Uncertainty can be frightening, but it can also be fertile. In a practice involving extemporaneous poetry we won't know the words until we speak them. Without the uncertainty there's no room for spontaneity, which is the heart of poetry created in the moment. The words come out on our breath -- anál -- and vanish into the universe. We look at what happens to see if there is an effect. And we note what happens (or what doesn't) and try again.
There's no escape from uncertainty, but we can embrace it and make it a friend.