One of the main fundamentals of sanding is to work with the grain of the wood, this is due to cross-grain sanding leaving scratches. The other fundamental is to use a sanding block, as this helps exert even sanding pressure.
Sand in long, even strokes alongside the grain of the wood. If you press too hard, then the pressure can cause gouging at the edge of the block. If your sandpaper wears smooth, then make sure to change it.
Sand the chosen sections of the furniture with each grade of sandpaper before moving on to the next and before each sanding, make sure to brush or vacuum the debris. If you get any dist caught under the paper, then this can scratch the wood.
Use a scraper for tight corners but use them carefully as they are extremely sharp and cause scratches if used unnecessarily.