My advice? Make a list of words, the “biggest,” most beautiful words you can find either in a dictionary or online, and then keep that list for reference.
Every time you write a poem or prose piece, try to fit a few words from your list into it. But don’t make the words seem forced; don’t just toss them haphazardly into your poem or prose just for the sake of adding a few “big words.” Instead, incorporate them smoothly so that the poem or prose piece still has a natural flow.
Keep repeating this process until you feel like you can use “complex" words in a natural, easy way that enhances your writing.
Best of luck!