assalaamu alaikum
mashaAllah sister, if you want to use more sophisticated words and widen your uses of vocabulary, memorizing the thesaurus is not going to work because that's just too much.
let me tell you what you can do and I'm sure that this will help inshaAllah....take an essay that you wrote and look at the dull and overused words. use a thesaurus and try to replace them with more sophicticated words. I'm not telling you to change every word, but start with a few and work your way up. The only way that you will memorise these words is by usage. Remember what you used the word for and what word did you use it to replace. With practice, this will become second nature to you and you wont even need to use the thesaurus that often anymore because after a while you will develop a better sense of how to use these words.
don't expect yourself to improve after doing this once or twice or three times. it takes a long time to go from a good writer to an outstanding writer....so just keep on writing and use your thesaurus whenever you think there might be a better word out there.
good luck!!!.......fill us in on your progress!