It's better to do it before so then when you start your finishing paper, you'll have everything done and only have to copy it.
Before writing anything, you must understand what you're writing about, or your topic. If you're writing a persuasive essay on why people should eat more kumquats, well you have to find out what kumquats are, firstly. Of course, you also have to understand your thesis. Without a strong thesis your essay will leave a lot to be desired
Just pour out your mind and start simple like rhyming with the ends of words