I would create 3 lists:
"advanced" verbs: think about the verbs you use all the time and find at least two alternatives for each.
"advanced" adjectives/adverbs: replace simplistic adjectives (big, little, fast, slow itd) with more advanced adjectives. Introduce more adverbs into your speech.
connecting words/phrases: obviously, as usual, in addition, consequently, although, however, in my opinion, initially, currently, finally itd
Then start introducing these more complex words into your writing/speech.