Below is an earlier answer of mine. I'm happy to answer follow-up questions if you have them.
TLDR: (1) Covering parties went out to screen digging parties; (2) Trenches were usually dug on reverse slopes, where the enemy didn't have a direct line of sight; (3) Once the first trenches were dug, it was fairly easy to improve and modify them under cover of darkness. Trenches also tended to creep closer together over time, and you could push a trench forward (called an approach trench or bombing trench) without exposing yourself above the ground.