Soak it in cold water overnight, then pound it with a mallet to break it into fibers. Then perform 3 or more extractions by boiling for some hours and pouring off the liquid. Let the liquid portions sit a day or two in the fridge to let any solids settle, and then decant. Finally reduce down to a proper drinkable amount.
I don't know which dimensions are 5 mm and 20 mm, but that sounds like a tiny piece of vine that may be better used as a table ornament. I don't know though. Typical dosages in terms of dry weight are roughly 30-120 g with the lower end usually being sufficient to potentiate DMT and larger amounts imparting more of the vine's own character. In traditional practice, the vine is the foundation which provides the primary effect, and DMT is an admixture added to "brighten" the effect. In the Western conception of ayahuasca, DMT is the primary effect and the vine is used as an accessory to make it orally active. These two different approaches to ayahuasca lead to rather different experiences and different outcomes, and a spectrum of experiences are possible by varying the amounts of vine and DMT. And of course, different vines and possibly preparation methods provide different alkaloid profiles which change the effects too.
YMMV with these things. Roughly speaking, larger diameter vines are usually more potent and may have preferable alkaloid profiles, but large diameter specimens are increasingly harder to come by because of they take many many years to mature. If I may brag, I have pieces which are like 75 mm diameter, which had to have been very old indeed. Now that I know how scarce this quality of vine is, I don't want to consume it. It's gorgeous to look at.
