With mono headphones, wire RI and R2 to the tip only of the stereo jack. Resistors RI and R2 might need to be changed, depending on the impedance of your headphones. For newer headphones that are between 39 and 100 ohms, try the 1.8k ohm value. For older headphones that are 4 or 8 ohms, try 1000 ohms. I built this project for last years Field Day.
Nothing is worse when working with headphones and having visitors.
Now the visitors have total independent control over the external speaker volume with no effect on your earphone volume. I use the box full time in my shack at home which allows me to use my boom headset and still allow independent external speaker level adjustment. I also only made provisions for a single headset.