It is possible to write a strong essay without having read any writing guides and without getting any help from others. But this is inadvisable.
1. At the very least, get the reactions of one student or peer reader and one reader above that level (a teacher, parent, boss, or professor). Try to engage them in conversation about the essay's strengths and weaknesses.
2. At the very least, read your essay out loud to catch typos and, more importantly, to hear the tone and flow of the essay. Try to read it in the presence of a peer and/or a superior, and have that person read it back out loud to you. Remember that the reader of your essay will read as the essay looks on the page, not the way you imagine it sounding in your head.
3. At the very least, put the essay aside for a minimum of 24 hours after it is "finished," don't even think about it, and then return to the essay with a renewed mind and a fresh eye. To make the most of that time, give others the essay so that they can suggest some editing improvements for you to consider. GradeSaver has extensive experience providing such advice; let GradeSaver give you specific directions for editing your essay to excellence.