Each painter has slightly different methods and preferences, but the pros all know the trade secrets.
You have to start with a perfectly smooth surface to end up with perfectly painted walls or woodwork. One pro tells PM that sander would be a more fitting job title than painter since he spends so much time pushing sandpaper. Sanding levels outs spackle or joint-compound patches and flattens ridges around nail holes. Sanding also removes burrs and rough spots in your trim.
Sand the walls from the baseboard to the ceiling with fine grit sanding paper on a sanding pole. Then sand horizontally along the baseboard and ceiling. Don’t put a lot of pressure on the sanding pole or the head can flip over and damage the wall. Sand woodwork with a sanding sponge to get into crevices.