It takes about six months to properly season wood, and the process involves cutting it correctly and piling it exposed to the elements. The seasoned wood should be the appropriate length for the stove or fireplace, which is usually 3 inches shorter than the length of the fireplace or furnace.
When the length is correct, split the wood to the correct width. In most cases, wood that is split three to six inches in diameter for a wood stove and four to seven inches for a furnace is a good size. Wood that is too large doesn't burn as well.
When all of the wood is cut to the correct length and width, lay it in a formation and allow nature to do its work. Best results come from a wood pile approximately four feet high. Leave enough space between piles to allow the sun and wind to get between them.
Some people like to cover the wood for seasoning, but it is not necessary. If the wood gets wet from rain right before it is time to store it, leave it in the sun for a couple days to dry. After about six months of seasoning, the wood is ready to use.