To make your own icon font and upload to Site Studio, the format of your custom icon font should be like so:

icons:
0:
name:"fa-fast-backward"
code:61513
1:
name:"fa-fast-forward"
code:61520

The name property is the a machine name of the icon and should be unique to the icon.

The code property is the decimal code for the icon (please note this is not the hex code of the Icon).

To convert a hex to a decimal there are a number of converters online https://www.binaryhexconverter.com/decimal-to-hex-converter

Icon font JSON format

icons:
0:
name:"fa-fast-backward"
code:61513
1:
name:"fa-fast-forward"
code:61520