Download Mod the Gungeon at: http://modthegungeon.eu/#download
Make sure Enter the Gungeon is closed
Open ETGMod.Installer.exe and press "Step 3: Install ETGMod"
Wait until it's finished, then close
Install the mod:
Go to your Enter the Gungeon Steam directory, usually located at:
"C:\Program Files (x86)\Steam\steamapps\common\Enter the Gungeon"
If there is not one already, create a folder called "Mods"
Drag AlternateStarterFix.zip into the Mods folder
If there's already a "mods.txt" file in the Mods folder, delete it
The directory should look like this: "Enter The Gungeon/Mods/AlternateStarterFix.zip
Then launch the game. If Mod the Gungeon installed correctly, you can press F1 to see the list of active mods.
Open the console with ~ or / to enter commands.
Type help for a list of commands.