Start by head to the following file location:

C:\Users\[USER NAME]\Documents\My Games\Sid Meier's Civilization VI

Now, make a backup of the AppOptions.txt file. Open that file and scroll down to the [Debug] section of the file. Look for a line containing the following text:

EnableDebugMenu 0

Change the 0 in that text so that it reads 1, and start playing the game like usual. Then press ~ (TILDE, THE KEY ABOVE TAB) to access the debug console.