Finger a low b flat and play long tones up the overtone series while holding this note. Change low fingerings and repeat. Seems easy enough... Not!
Sigurd Rascher's book is essential because it teaches you embouchure control and ear training. An added bonus is that you get to extend your effective range on the instrument by an octave or so.
In my view, intermediate players should start this type of training as soon as they conquer the basic tone production challenges. I have seen many a player literally "take-off" as a result of time spent in this book.
While it may seem tempting to just the learn the fingerings and skip the exercises, don't do it. I tried the same thing years ago and picked up bad habits as well as out of tune fingerings.
Pay your dues and follow the book. Ted Nash and Larry Teal have written books which also address the "falsetto" range of the saxophone.
Get them...