Log in to Fathom.Video and navigate to the My Calls page: https://fathom.video/home
Click on the call that you wish to highlight, so you're on the call details page.
Hover over the transcript. Click on the blue plus sign to the left of the transcript that you wish to highlight. Click on the highlight type that you want to add.
You can drag the highlight up or down to include more of the transcript.