Unless you are more experienced, scratching is going to happen when you sharpen your blade. By studying the marks you leave, you can get an idea of how uniform your sharpening strokes are or aren’t. The closer you can keep them to the blade’s original shape and angle’s, the better off you are. If you’re leaving scratches near the back of the blade, you’re holding the blade way too flat against the stone. Over time you’ll get the feel for how to hold and stroke the blade against the stone and you won’t scratch your blade during sharpening. One way to get around this problem is to use an DMT aligner sharpening system. This will guide the blade and keep the angle from changing while you sharpen. DMT has a system like this for under $50. Well worth the money. I have personally tested these angle systems. They are a little slower to use but work well to set the desired angle and produce a razor sharp edge without so much as a single scratch to the blade. Another tip is to roll the knife on the edge to feel where the flat of the edge is. Then you sharpen on that flat edge angle.