How exactly is the wood fossil process?
there are actually more than one process, but in the case of high detail wood fossils the process is the slow filling in of all the spaces in the wood (filled with water in life) with dissolved minerals from the groundwater. this is called permineralization. Of course this only works is the wood is buried so it cannot rot while this is happening. Since permineralization happens on the molecular level is can preserve great detail as the minerals form around the remaining hard tissue. 

This video is a decent representation,