I learned by going down to the local park and just sitting there listening and watching the ducks. Seeing what calls they use in different scenarios really helps put it all together.
My dad always told me to call only at wing tips and tail feathers. If they were right overhead or coming in there is no need to call them.
Too much calling can give away your position too, so be really careful about movement.
I used to practice in my truck on the way to work in the mornings too. You do get some really funny looks from other people going down the road.
