placement of your bottom lip is key as well as allowing no air to escape. If you duck call it is the same principle as a feed call. You can roll "TUKA" or "DIGA". try to start the pitch HIGH then as you proceed roll the pitch down. You dont want to go from low to high back to low (which will actually be your natural habit to do)
It is sorta hard to explain in TYPE but Youtube should have some stuff that actually shows it. Good luck, it took me about 2 years to actually get good at it but man if you can it can be a deadly call.