I make stupid mistakes everytime while solving Quant questions , somehow i know the method and know how to get to the answer but simply mess up in the end. How do I improve upon that?????

Practice. There are two things. 1.Concept Clarity 2. Speed . While you solve questions ultra fast your concept clarity is tested to max. If your concept is absolutely clear, when you solve in higher speed. It will cooperate. So unfortunately practice is the only thing you can do to rectify this. No Magic available.