Most writers find that it is nearly impossible to craft an essay by beginning with the introduction.
The best leads often develop during and after writers have written the remainder of the essay. Maybe a fantastic introduction or conclusion is caught floating around in the middle of your rough draft. More likely, however, it is in these later stages that you have a good sense of the way your essay is shaping up, all the way to the nitty-gritty details. Since beginnings and endings can be the most challenging and important part of any piece of writing, you will want to take advantage of a completed rough draft. Part of the reason why introductions and conclusions are so difficult is that writers tend to worry about them too much.