How should I conclude an essay based on "fair is foul and foul is fair" from Macbeth?
Let us remember that the conclusion of an essay has to restate its main thesis and briefly round of the essay as a whole through revisiting succinctly the main points of your main body paragraphs. You should not include any new information in a conclusion and you should also not provide a summary of your entire essay. Lastly, a conclusion should round off the essay in some notable way to make it linger in the reader's mind.
Thinking about these aspects of a conclusion with regard to your essay title, I am sure that one of the key elements you have developed in your essay is the way that this famous quote relates to the key theme of appearances vs. reality, which of course is so important in this play. In the witches themselves, in the character of Lady Macbeth and in other characters this theme is obvious. Your conclusion then will want to restate your main thesis statement, your main argument, whilst touching briefly on such examples of proof. Without knowing your thesis statement I can't really give you a specific example, but here is something for you to work with.
The ubiquitious Shakespearian theme of appearances vs. reality is thus encapsulated in the witches' saying, "fair is foul and foul is fair." Its significance is shown to apply to the witches, to Lady Macbeth and to other characters in the play such as Malcolm. Through the example of Duncan, Shakespeare shows us the dangers of judging by appearances alone and what tragic mistakes we can make as humans if we are not aware of how easily appearances can be constructed.
Hope this helps! Good luck.