Polygonum cuspidatum root processing

I found Japanese knotweed, I dug up for it's root, I cleaned it up, but I don't know what to do next. Someone please tell me how do I transform that woody root in a decoction or in a powder intended for capsules filling. I need it for helping my 19 old year son who has chronic Lyme borreliosis (ocular and neuroborreliosis, arthritis).
Another thing I don't know is about the dosage needed for both forms (powder for capsules and decoction).
Thank you very much

[JackieCalifornia]

I don't know that we have any herbalists here --

I have a book, "Healing Lyme", by Stephen Buhner, subtitle:  "Natural Healing and Prevention of Lyme Borreliosis and Its Coinfections."  It's available to order online, I think I got mine from Amazon.  He seems to be by reputation a highly respected herbalist, but even he does not describe how to process the raw plant; he gives sources to buy it already prepared.

Knotweed appears to be the active ingredient or source in Resveratrol, which I see in almost every vitamin/supplement shop I go to.  Might be easier to buy a bottle.  It's not expensive.  ... and with doing it yourself, you might not know which parts of the plant could be toxic or whatever.

fwiw.  best wishes to your son, and to you --