These instructions are for single player and coop games. For dedicated servers see the other pinned posts in the discussions.

To play using Project Eden or Reforged Eden, simply press the subscribe button on the main workshop page for the scenario, then start Empyrion. If you already had the game open, close it and reopen it. You may need to allow Steam time to download the scenario files to your PC.

Go to New Game, choose Single Player or Co-op, and then select Scenarios. Select Project Eden or Reforged Eden from the list of available scenarios. Set up your game difficulty settings and choose your starting planet and game mode (survival or creative) then press start game. You will know you're playing the scenario because it has custom loading screens and you will have a Project Eden Guide item in your inventory.

The scenario will only apply to new games specifically started using the scenario, it will not affect any of your other save games or ability to start a game using a different scenario or the default survival mode.

Internet access is required to play saves using the scenario. If you want to play without internet access, see the Project Eden Configuration Guide for instructions on installing a local copy of the scenario:
https://steamcommunity.com/sharedfiles/filedetails/?id=1703779262

Co-op mode allows your friends to join your game. To play Eden co-op it's best to first start your game in single player mode, then exit it and resume the save game as co-op (press the little arrow in the bottom right and select co-op when you go to resume the save game).
This is because due to the size of the scenario it might not load properly if you start a brand new save game as co-op.
If that still does not work you can then install a local copy of the scenario following the directions in the guide above and then starting a new save game.

If you are playing on a server that is running the scenario, you do not need to subscribe. All needed files will be downloaded to your PC from the server automatically the first time you connect to a server.

Project Eden and Reforged Eden are more demanding on your PC's hardware. At least 16gb of memory and Windows 10 is recommended, as well as a decent CPU and graphics card.

If you have troubles with the scenario, see the other pinned posts in the discussions here.
The game developers are not responsible for custom scenarios, you need to first seek troubleshooting assistance here.