You would be using it on an empty font that is already added to a project - GameMaker won’t let you “add existing” resources that aren’t in a project anymore.
Thank you for your help Yellow! Very much appreciated, I'm excited about it.
So I followed the steps of the tutorial, dragged the fnt file to BmFontToYY23 and got a .yy file.
First I tried dragging the yy file into the project and got the error
Error: Could not find a project for the resource file with the file created by BmFontToYY23
Then reading your response I created a font with the same name and replaced the files inside with the ones generated.
It came out garbled sadly.