Core problem: the game supports Workshop at launch, and subscribed maps show up in an in-game Workshop list, not mixed into the official campaign. The part people miss is that the browse page lives in Steam, not inside the running game. The in-game menu is just your subscriptions.

1Find the Workshop

2Subscribe and Wait for the Download

The restart step is the one most people skip. Subscribing while the game is open and then checking the in-game list without restarting is the number one reason maps "do not show up."

3Play It In-Game

4Key Decision Points

Browse sort
Popular vs newest
Start with Most Subscribed or Best rated if you just want something that works. New maps can be unfinished or tagged joke content.
When to start
Official campaign first vs Workshop immediately
Workshop is unlocked at launch. You do not need to finish the story. Many players jump to community maps after a few official levels because the official set is short.
How many to subscribe
Subscribe only what you will play
Unwanted maps still download and clutter the in-game list. Blind-subscribing everything makes the menu hard to navigate.

Common Mistakes

If the Map Still Does Not Show

Once a couple of subscribed levels work, the loop is just: browse Steam Workshop, Subscribe, restart if needed, play from the in-game Workshop menu.