First, you need to install the Software Development Kit (SDK). Go to your Steam client, then click Library to open the menu and select Tools. Find the Sid Meier's Civilization VI Development Tools entry, right click it and install it. Note that you do not need to install the Sid Meier's Civilization VI Development Assets package. After the installation has finished, run it and select "ModBuddy". If it asks you to install some Visual Studio stuff, you must first download and install that one too. ModBuddy launches into a pretty empty, black screen: