How did Trenches during WWI work?

They were built all the time, always before combat, often times all you were trying to do is advance from one trench line to the next (which was often occupied by the enemy). So noone was building them "during" combat or they would've ended up dead. There was no purpose of building a trench after fighting either, because you were already in one if you were successful in that day's fighting. Imagine a football field is the battlefield, and every 10 yards you have a line that runs across the width of the field marking the 10 yard line, the 20, the 30, and so on. In this type of battle it's the team that advances the ball into the other one's goal line that wins this kind of battle....and it was hard fought for terrain.