The wood needs to have no bark, and be very hard. If you can press your fingernail into it, don't use it.
Boil it if you can fit it into a pot. It will stink. Change the water often. Then you'll need to soak it until it will sink and stay submerged. You will need to change the water a lot. It may leech tannins for months, there's no way to tell until you're doing it.
You can cheat and speed up the sinking process by boiling it and then soak it in icy water, then boil again, then icy water again. Do that until you just can't make yourself do it again. Boiling it will open up the pores in the wood, and submerging it in an ice water bath will snap them closed again quickly, trapping water inside.