5. By exploiting the salient as a form of flanking. It's how the Germans werre finally broken, and the germ of the idea was obviously there from the start, it's the understanding of it which was lacking. You stage a limited frontal assault to the point at which it is about to stall, and then exploit the containing flanks, weakened by the push. Either the troops holding the edges of the broken line retire, causing chaos in the area you're targetting, or they get cut off. Let's look at the right flank exploitation. Youe either keep going, rolling up the opposition, or you penetrate and then turn left. The troops there, probably either recuperating or second-level, will be focused on what's happening to their right, the head of the original salient, and so get flanked by the attack, and so it continues, much like following the mortar lines up a wall, where the bricks are the enemmy strongpoints.
In fact, it wasn't necessary to go that far, as the extra problem the Germans had is that they were on the outside of the curve around the north-east of France, stretching them further. The problem the defender has is that he must defend everywhere, and has to win each time: the attacker only has to win at one point and once. The Germans were stretched thin, broke, and rolled up.