You need lots of diaphragm, just like a sinlgle reed duck call. Pretend like you are blowing bad breath in some one's face. Huugghhh
You also need to control air flow with your tongue to get good sharp clucks and honks. To cluck I say "whit" with a growl (diapragm) at the "wh" part. This puts your tongue on the roof of your mouth and stops the air flow. You can change tones of your clucks by using more or less air and growl. This will help you sound like more than one goose.
Make sure to create backpressure by tightly holding the end of the call and cupping the end with your free hand. You can change tone by doing this. Good calls dont require much back pressure, but it still is useful.
To do the "comeback" call say Too-ahh into the call. The read should break on the "too" and "ahh" should be a moaning sound.
I used the hunter's specialties audio tape to learn, but I'm sure any of the mentioned videos would help a lot.