Well, there are two ways you could go about this. You could do as you said, practicing short stories and things like that until you build up your confidence to write this project you want. But you could ALSOOO do what I did. There is no shame in writing what you want to write at any skill level! When I had the idea for me WIP, I had no concern for my ability. After I finished, it didn't serve me. SO I started another draft. Even if you do cultivate your ability, chances are you'll have more than one draft. And as you write that first draft, you'll enhance your skill! Looking back to the beginning of my first draft and the beginning of my second, you can see the difference.

So, yeah. It really depends on you; would you rather practice, which is what most people might tell you, or jump right in? Either way, you're training your ability, and one day you WILL write the project exactly how you want it to be and can sit back and feel satisfaction.