save your money and listen to the real ducks for practice. There are techniques as to how to blow into a call and what to say into the call that can be picked up in DVD's or books, but all you need to figure out is the single quack. turn your call around, and blow into it that way so you can hear what your diaphram/voice is doing on each quack. you can hear if you are using too much voice or not enough diaphram. i got started using an OOOOTTT sound into the call. but nail that single quack. master that first.
http://www.ducks.org/hunting/duck-calling/index.htmlForget feeding chatter, waste of time, once you get that quack down, then get out early somewhere that is holding ducks and listen to what/how the hens are calling. Calling especially on public land is way overdone, and the more life like you can get it, the better off you will be. you could maybe even search for mallard sounds on youtube or something to hear them and then try to reproduce that sound:
http://www.ducks.org/hunting/waterfowlGallery/12/index.htmlespecially listen to that fast cadence at the end of the clip, i use that a lot, not as long, but a faster cadence than is usually taught.
during my years at UW, i would walk by drumheller fountain, which was stacked with mallards and heard quacks and series of quacks that were not in any DVD's or books. Then on hunt days, I'd try to get to the blind early and listen to what the ducks were doing in the morning. those small changes added a great deal of success in my calling/hunting.
All just in my opinion, good luck