The enlisted soldier was censored by an officer in his unit. It was considered an unimportant job and often someone like the chaplain or the dentist would get saddled with the job. If the enlisted man did not want his officer to read his mail -- if he had been giving him a hard time, let's say -- the soldier could use what was called a 'blue envelope.' The writer would certify that there is nothing in here that shouldn't be and the letter would go up to the next level where it might be looked at a little more kindly.
If the section they wanted out was very big, they would confiscate the letter. If it was small, they cut out the words or obliterate it with ink. If they had to use special chemicals to check for invisible writing -- something they did when they suspected a spy -- they would confiscate the letter because they didn't want people to know they were doing it.
The censors returned very few soldiers' letters. They confiscated them; they didn't send them back. They didn't necessarily give the word back to the soldier that his or her letter was withheld. It depended where it was stopped and how fast the troops were moving.
From the soldier's perspective, they often didn't know if it was going to get through. The soldiers were all given guidance on what they could say, so one would think they would know how to avoid getting their mail intercepted, but not all did.
An overly informative writer might be talked to, because it's important. We don’t know of any soldiers who were severely punished for what they wrote in a letter. It wasn't considered an overt act of sabotage; it was considered carelessness. Bill seems to be careful to follow the rules of censorship in his letters. It is unknown if any of his letters home were altered or confiscated.