I practice in the truck too. Start with a basic quack. Perfect it... Start to work on a feeder call. After you have learned those basics your mouth with have the muscle memory and practice with pressure and volume to work into greetings, hails, comebacks and pleads.
When you hunt, only do what you are good at. If all you can do is a good quack then don't attempt to hail or you spook them off. A good basic quack will let them know you are there and will get them working. Focus on what you can do well to start off with and the rest will eventually fall into place. It can/will take a few seasons to learn to sound like a duck and then apply that to bringing birds in.
Remember that less is more especially with late season birds.
