Screw it to a piece of slate and bury the slate in your substrate.
Boil it for an hour or so then plunge into a bucket of cool water and weigh down. Wait for it to cool then boil again and repeat the process until it sinks on its own.
I found some cool driftwood at the lake and I'm going to put in in my 30 gallon tank. I've already boiled it, but the wood still floats. How can I make it stay at the bottom of the tank?