How do you season wood?

What do you mean by season? To prepare for further milling or to 'age' it for a desired finish? First of all it may depend on the type of wood. Regardless, to season or store raw lumber for milling and indeed to store milled lumber prior to use you can stack the lumber using 'stickers' between each layer. A layer dimension is dictated by the length of the lumber and the thickness of the stock but you can control the depth of the layer. It is important to use stickers, which is just a piece of wood, generally 1x1 or 1x2 and as long as your layer is wide, placed across the layer about every 1' to 2' along it's length. Stacking wood this way allows for air circulation which will help dry or season the wood prior to milling and to keep milled stock stable prior to use.

To age wood depends on the type but generally there are two DYI ways, both of which requires diligence on your part because of the chemicals involved. The first method is fuming where you introduce the wood stock into a chamber fumed by ammonia. Design methods and plans are on the internet by searching 'fuming wood'. This method works especially well for oak. Another method, one used for conifers but especially dangerous, is to wash the finished stock with lye water. It provides a very desirable warm, aged orange patina but the lye water method is very dangerous.