On MacOS

  1. Open file in Preview.
  2. Click and drag mouse to select desired portion of file (pay attention to aspect ratio tool tip, ensure selection is a square).
  3. Crop image (Tools → Crop).
  4. Adjust Size (Tools → Adjust Size...).
    • Ensure resolution is 72 pixels/inch.
    • Set height/width to 32 pixels.
  5. Export as ICO file (File → Export...).
    • Hold option key when clicking Format selector to show all possible options.
    • ICO filetype may be labeled as "ICO" or "Microsoft Icon".
  6. Move favicon.ico to website source root.
  7. Add <link rel="icon" type="image/vnd.microsoft.icon" sizes="32x32" href="favicon.ico"> to html file <head> attribute.