The sanding sheet you got should have some numbers on it: 40-60-80-120-180-240 etc.
You will need to go to a lower number than the sheet that caused the damage to get the scratches out. Work through the different grades to a high number as the higher the number the finer the grit ( and the harder to remove scratches ) but equally the more you remove the less of your neck is left, so its a balancing act!
Don't use the sheet with your finger, get a small wooden block, wrap or stick some of the sanding sheet on it and gently sand off the scratches but don't just sand in one place or you'll cause a flat spot. Using fine steel wool will give you a satin finish but with any abrasive media, the more you sand, the more you take off the original neck finish....!