The length of time to season a piece of wood will vary due to variations in wood density and size of cut, the drying process and storage.
What best Natural air or Kiln seasoning ?
Both methods have their advantages & disadvantages. It is about looking at the product and choosing what is best.
How much warping will the timber go through?
It depends on a number of factors. These include:
position that the plank was saw, the storage, seasoning, species, dimensions of the lumber.
How do I stabilise the wood ?
Session timber, cut to rough size and let it rest several weeks or months extra seasoning. Then machine to final size, apply a finish to all edges to prevent any further moisture gains or losses