Answering the question provided, it would depend on the kind of sturdiness you would like to imbue upon the object. Silk is a far shot from being leather, more likely to submit itself to wear and tear, and that would be moderate structural modification, thereby needing two steps.
But that is because I am in the belief that silk, the real silk, not the synthetic ones available in the modern world, is quite a precious and delicate material for clothing and drapes.
Gossamer would know more about saddles than I do; my initial take on a saddle made of silk, though magecrafted, is that it sounds like a waste of resources. I have ridden my share of horses, and I believe there are a lot of other saddles that does not compromise durability and comfort. Unrequited advice, but maybe you can just ask for a "personalized saddle"? You'll save up on miza that way