The easiest way to do this according to the Bitcoin Core Wiki is to append the datadir option after running bitcoind to launch the client according to the following syntax:
example provided thusly:
Also, don't forget ...
Your understanding of the commitment root is correct. Taking advantage of Haskell's laziness, feel free to put undefined into the signature when computing the commitment root.
At the moment, the witness root is not used in the protocol. There are some ideas on how to use it in more sophisticated protocols, but those ideas haven't been realized yet. ...