First of all, you should take notes of all main events of the story of your novel. It will help you to briefly mention all the main points of the novel in the prologue, which is generally considered as a short story of the novel.
However, you should avoid writing any point that could lead to resolving of various issues which are in the novel. A prologue should be free from the element of conflict resolution.
Give a proper beginning, middle and end to your prologue that could entice the readers to read the whole novel with a greater level of interest. Wind up your prologue in such a way that hooks the reader for wanting to read to more.
You can start writing the prologue of your novel in an innovative way by mentioning something interesting and then gradually relate it to the story of your novel.
Make sure you do not forget to add background information of the novel in your prologue. It will help readers to understand the context of your novel and they will have be able to identify the subject matter or the main story.
If you are confused about the style of writing in a prologue, then you should read as many prologues as possible. You can also discuss the main contents of your novel with your literary friends as sometimes it helps in getting new ideas for writing a great prologue.
If you could not utilise a phrase in the main story of the novel, or could not find a place where you could make it fit, then write it in your prologue to fulfil your purpose.