How do I change VS Code Settings to use JetBrains Mono Font

Open the VSCode Settings (Code -> Preferences -> Settings), then add to the end of the User Settings JSON file:

  "editor.fontFamily": "JetBrains Mono",
  "editor.fontSize": 13,
  "editor.fontLigatures": true

Building on what @Grekkq and @Gyliph provided, this is how my settings look:

    // Set the font
    "editor.fontFamily": "'JetBrains Mono', Consolas, 'Courier New', monospace",
    // Set the font size
    "editor.fontSize": 13,
    // Turn on font ligatures
    "editor.fontLigatures": true,
    // Set letter spacing
    "editor.letterSpacing": 0.4,
    // Enable smooth scrolling in the editor
    "editor.smoothScrolling": true,

The trick to adding JetBrains Mono to the current list of font families was to wrap 'JetBrains Mono' in single quotes (since there is a space in the font name).