combining stereo channels

I want to combine existing stereo channels into one channel. Then I want to have the combined channel copied to the left and right stereo channels for monaural playback from both channels.
Please explain the steps and menu items used.
Thanks for your help.

Select the tracks you want to convert from stereo to mono.
Select Tracks > Stereo to Mono.

You don’t need to do the second step. A mono track will play back equally through both stereo channels.

– Bill