There is no special way to notate a thumb-under. You can use the thumb-under whenever you need to press down on a fret that is too far away from another fret further up the fretboard.
The thumb-under makes it possible for further stretches be made on the fretboard.
To do a thumb-under, move your wrist under your guitar's neck and place the side of your thumb on the string and fret. The pinky can then be placed at a fret further up the fretboard.
If you feel the thumb-under position is uncomfortable, try moving your wrist further out and away from yourself. This gives the thumb more room to avoid touching unwanted strings.
There is no continuation to this technique.