How did soldiers mail letter during World War 2?

Mail was delivered by the civilian Postal authorities to the Military postal authorities. The military then handled the mail delivery to army units, navy ships, etc. The mail would have been carried to/from the trenches by soldiers. Most military units have sailors or soldiers that are "mail clerks" that sort incoming mail & gather out-going mail. These clerks would distribute the mail to the units, the leaders of the units would get it to the soldiers. If you have seen American War Movies, many show the scenes when the soldier responsible for getting the mail to the troops calls out "mail call", and soldiers gather to get mail.