Start sanding with lower grit papers to remove larger scratches.

Follow with the next finer grit, up to 320 grit.

Example: Start with 120 grit, then 220, then 320.

Each jump in grit removes more scratches from the previous grit.

Even "clog free" sandpaper will fill up with sanding debris. Clean the paper frequently with a folded paper towel to remove debris and prolong the sandpaper's effectiveness.

Sandpaper becomes dull with use. Replace a worn piece with a fresh sheet and progress will much more rapid.