This is a very easy one, as the description says, you need to download a map or scenario from the steam workshop. To do this:
1. Close the game
2. Go to the game launch page in your library.
3. Near the top of the page, you will find a section called "Workshop", open it to reveal the workshop.
4. The most popular maps and scenarios of the week will appear, you can also find something more specific.
5. Download a map or scenario.
6. Once downloaded, reopen the game.
7. Locate the downloaded item. If you downloaded a map, you will find it in "Maps". If you downloaded a scenario, you will find it in "Scenarios"
8. Once located, start the map or scenario. When the simulation begins, the achievement will pop.
If you've already uploaded items to the Steam Workshop, you can use those items too.