Flux is applied to the areas you are going to solder. Such as inside the fittings and a bit farther up the end of the tubing than fits inside the fitting. You only apply flux to already cleaned copper tubing and fittings. You clean them with sand paper, a wire brush or emory cloth until the shine and all the coating is gone and you’re down to bare copper. Then you apply a thin coating of flux. The flux should be ‘lead free’ if you are soldering potable water lines.