Here is the thread where I talk about the process.
Vanda Emergency
I just did this to a Vanda that I got from Exotic
Orchids of Maui. It had been grown half way to maturity in a black plastic basket, with most of the very extensive roots growing straight down out of the basket. After cutting the basket away, I cut off the thinner roots, those about spaghetti sized, as there just wasn't going to be enough room for all those roots in a 4" azalea pot, even when bent like spaghetti. Worked like a charm. Even managed to get he Vanda centered in the small pot.
I don't like cutting off living roots ever, but I have a theory that an orchid grows as many roots as it needs, and when you grow a plant without a mix, as many far eastern growers do (empty basket culture), the plant has to grow many more roots to soak up enough water and nutrients during the brief time it is watered.