How to smooth a wall.
The basic principle is “sand the high spots and fill the low spots”.
A wall finishing professional like a drywaller or flatterer may have differing methods or techniques than are described here. But then, they probably have a higher level of skill than us. They may still find this amusing.
I’ll use an interior drywall wall as an example as it is most common and easiest to work on.
Sand the surface texture with medium grit sandpaper or sanding screen (100 grit cuts quickly and doesn’t leave deep scratches that may be hard to cover). When your sanding stops making reasonable progress, stop. No point in pressing a diminishing return.
Next, you are going to use premixed drywall mud and apply it to the wall. Use a mud pan to hold your mud and spread the mud with 10″ or 12″ drywall knife. Hold the blade of the knife at an angle. Like you would buttering bread or frosting a cake. Apply the mud either in horizontal strokes or vertical strokes for this coat. “Put it on and wipe it off”. The idea is to fill the low spots that you couldn’t get to sanding not to build up a thicker surface. It won’t be smooth and the mud will shrink. Let it dry. Sand the mud flush to the existing wall texture.
Repeat the first application step going perpendicular to the previous coats strokes. Left and right instead of up and down. Or, vice versa. Let dry. Sand.
The wall is probably not smooth at this point. You can decide if repeating the first two steps is required or just going over rough areas with another coat in whatever direction works for filling the imperfections.
Positioning a light to shine down the wall at a tight angle will highlight imperfections and make touching up easier to do.
Look at the wall after you prime it to see if it is going to be satisfactory. Better to do your additional touching up now instead of after the finish coats. A flat finish will not show imperfections as much as a satin or gloss finish.