The Demo comes with two 'Quickstarts', with 57 more available on Steam Workshop.
England for Football Manager full simulation mode, and England, Italy Spain for Football Manager Classic.
Select "Leagues" from the Start Screen on the Football Manager 2015 Demo.
The Steam Overlay will now appear.
Select the League in the Workshop you wish to download and click "Subscribe" to download.
Press ALT+TAB on your keyboard to close the overlay and the demo league will download. When completed, just go to Football Manager or Football Manager Classic and select the league you want to play.
Each demo save game allows for 6 months of gameplay.
Go to Documents > Sports Interactive > Football Manager 2015 Demo > games
Copy the demo save game into Documents > Sports Interactive > Football Manager 2015 Demo > games.
When you load the full game you can continue your save as normal by going to Load Game.
Go to the Start Screen in game.
Open the Steam Overlay with ALT+TAB.
Go to the League you have downloaded in the Workshop.
Click the subscribe button to unsubscribe.
Then when the button says [Subscribe +] again, click this to begin downloading the demo.
In the Steam Overlay, click the Login button in the top right of the window and log in.
When going to the Steam Workshop in-game, click the Collections tab.
In here you will be able to download the Complete Demo Collection, and download the additional set of leagues.
Be aware that the collection is a combined 1.2Gb, so may take time to download depending on your connection.
This is because Steam Workshop is downloading league files individually and the dialog appears for each individual download. Please wait while this completes.
If you subscribe to Steam Workshop files outside of the game they should download automatically. However, the game needs to be run in order to finish the installation of the download.
If no downloads at all appear then go to the Steam Workshop in-game and unsubscribe to the items you subscribed to. Use the Steam Overlay in game to subscribe to files on the start screen, and they will download.