Obviously they fought in trenches, but how did this trenches get there? Do both sides send people out to dig trenches, then a battle would break out there in the near future? How did they decide where to put the trenches?
I'm happy to answer any follow up questions, but my recent answers below to similar questions may help you out. The first touches on the transition from mobile to trench warfare and how open combat degenerated into static fighting. The second discusses digging trenches more specifically.