Look up examples of the new word / phrase in real sentences e.g. Oxford dictionary, frazeit.com

Note how they are used and collocated including the tense, prepositions, word order, synonyms.

Produce your own logical diagram for how to use the phrase e.g. if you wanted to use the word "mistake", you might find a sentence like this: 

a) "I made a mistake when writing your name."

b) "I never make mistakes in maths."

Your forms could be : 

a) subject + make + (a) + mistake(s) + when + gerund clause

b) subject + make + (a) + mistake(s) + in + noun (field of study)

Later you might then see sentences which use "mistake" with "about", or followed by an independent clause.  Then you could improve your forms.

Practise by writing your own sentences including the new word / phrase and post them for correction on Italki.

Improving one's writing ability takes a long time. One idea is to write down all the vocab that you believe is "advanced" and create your own sentence with it and review that word and sentence in a SRS system. Another way is with Microsoft Word you can right click on a word and look for synonyms. Instead of saying "boring" it will show you more descriptive words, such as montonous, lackluster, tedious etc. Be careful though, as these words are not synonomous, each has its own specific meaning.

So at the end of the day, reading more to build even more vocab and give you an exposure to how each of these words should be used. Also, try not to conflate being able to use advanced vocabulary with good writing, if you have a message and can convey that message with "simple" vocabulary in a clear way, that is good writing. 

Ah sorry Maggie - I forget exactly what SRS stands for, I believe it is someting along the lines of Spaced Repetition System. The idea is that you review the world only when you're about to "forget" it, and then when you review it at that point it helps settle it into your long term memory. For example, your first review may be one day, then three days, a week, a week and a half etc.

I think the most common one is Anki, if you google that you should find what I'm talking about :).

You can read more and more novels , this site may help you ( readtheory.org ) . 

Good luck,

Read different kinds of books.....