Don't. I know the use of fancy vocabulary may seem like a sign of sophisticated writing, but it's not. Every fancy word you use makes you prose less accessible to readers. The only reason to bring in a fancy word is if you cannot express the idea you need to get across using simple words. The hallmark of a good writer is that they can get complex ideas across using simple words. Practice that.
The paragraph above is perhaps not the best example of the advice it gives. Did I need to use the word "hallmark" for instance? Some readers may not have an immediate recognition of what "hallmark" means. (A hallmark is an imprint that a craftsman makes in precious metal to indicate who made the object. My wedding ring was custom made and has the hallmark of the goldsmith on the inside.)
I could have used simpler language for this. I could have said "sign" instead of "hallmark". Then my meaning might have been clearer to more readers. Or I could have simplified the sentence even more and said, "A good writer can get complex ideas across using simple words."
There is a saying that gets attributed to a number of different writers, "If I Had More Time, I Would Have Written a Shorter Letter". Our first drafts tend to be lengthy and verbose. They tend to use all sorts of complex and difficult words. If we have time for a second draft, we can and should make them shorter, and use simpler words.
If you read avidly, as you should if you want to be a writer, then your vocabulary will naturally grow over time. Your challenge, as a writer, is not to incorporate all that vocabulary into your writing, but to resist doing so as much as you can so as to make your writing as simple and easy to understand as possible.
Hmmm... I wonder if I could recast that first sentence to avoid the word "avidly"...
EDIT: Interesting finding: "Interestingly, using long words can result in readers missing shorter words that follow them, which can greatly affect text interpretation. http://centerforplainlanguage.org/what-is-readability/