Create a file in Notepad. Write some functions. If you want weapons/vehicles, start using Add-Ons/Weapon_Gun.cs and change all the names in the file. File>Save As... All File Types, save to Add-Ons as whatever. Start Blockland, enable it in the Add-Ons list, start a server (single player while testing) and watch the console for red Syntax Errors as it loads the Add-Ons.