When the narrator says , "If still you think me mad you will no longer when I describe the wise precautions I took for the concealment of the body" He tries to convince the reader he is not insane (mad) by all the precautions and effort he has put into the concealment of the body.
His explanations not only make the reader realize that the narrator is very unrealiable, but realize that they are quite insane depsite the constant "I'm not mad".
hope this helps :)
