To install the Level Editor, download the package and start the installer. Please make sure the installation directory is the same one where you installed the retail copy of Shadowgrounds. In most cases the installer should automatically know where your Shadowgrounds is installed.
After installing the Level Editor package, check the Shadowgrounds folder and you will see a folder "Mods" there. Inside that, there's a folder called "mymod". All of the editing you do for your mod/missions will happen inside this folder. You will never need to replace the original Shadowgrounds files, which means you will always have a backup copy if something goes wrong.
You should rename "mymod" to something else (a good idea would be to rename it to match the name of your mod. Use only lowercase letters, and no spaces or special characters. So for example "mybestmod" is ok but "My Best Mod" is not.
The name of your mod (shown in the Shadowgrounds Launcher) is defined in the "description.txt". Make sure you write something here - otherwise other gamers will not know what mod they are trying to launch. You can use uppercase and lowercase letters, numbers and some special characters as well.
The Shadowgrounds Launcher has been updated in the Level Editor package and in the official patch. This means that all Shadowgrounds gamers will be able to launch mods. Gamers can read the instructions on the New Missions & Mods page but here are some simple instructions when you test your mod for the first time.
Simply start the Shadowgrounds Launcher, select the mod (whatever you named it in the "description.txt"), and activate the mod. The Launcher will relaunch itself (it will not save the settings before relaunch). Now you can launch the game and the mod will be playable.
From the game menus, choose New Game and the mod loads. If you have not made any editing yet, you should see an empty loading screen with "My mission" header and text. When the mission has loaded, click Start. The game will launch the mod - which at default is a sample mission that showcases some of the possible features.