To prepare your Figma frame for exporting to a favicon package using Favvy, your Figma layer must be a square aspect ratio, where the width and height are identical (eg. 512x512 pixels).

The quickest way to do this is to create a new blank Figma frame on your page, set both the width and height for your new favicon frame Figma to 512 pixels, and then drag and drop the icon you'd like to use for your favicon into the new Figma frame you've just created, and resize it as needed to fit the 512x512 frame.

For the best results, you should use SVG vector layers inside your Figma frame to ensure that the exported favicons are as sharp as possible.