How did you learn radical acceptance?
The hard way

I think the concept originates from DBT via Marsha Linehan (who developed DBT and is herself someone who has been dx'd with BPD), and she brought it over to DBT from Buddhism. I think of radical acceptance as accepting life as it is, saying yes to what exists, not trying to change it. There may be a lot more depth to it than that, especially when threaded together with the concept of dialectics (e.g. two seemingly opposite things can both be true).
Applying that to family has been a challenge because of the deep yearning to feel validated from the same people who can't.
For me, radical acceptance is the very profound recognition that my family members don't see me, therefore they cannot validate me. I am one dimension. They are tolerating me in two dimensions, but mostly because I have found ways to make that dimension tolerable using different skills and boundaries. The third dimension is a mystery to them and in all likelihood is too threatening because it would require intimacy, something that is largely foreign and to be fought at all costs. Radical acceptance is giving up the dream that this will ever be otherwise.
Related to this is something that may go together with radical acceptance, or at least make it easier to manifest the way Riv3rW0lf described, as a feeling -- is body-based trauma work similar to what Bessel van der Kolk talks about in The Body Keeps the Score.
There are multiple paths to get there, but I did somatic experiencing therapy (SET) work (
https://traumahealing.org/), which focuses on body-based trauma therapy. It did powerful things to help undue triggers buried in my nervous system. There is radical acceptance, and then there is healing. For me, the two go together.
Where/how did your learn "dimensions" of people? This is a wonderful and helpful paradigm!
It's just a way I explain to myself what I'm doing as I work at a relationship in which there is both radical acceptance and abuse.
And to be candid, it is a nonstop challenge
