stereo to mono

Leave ground common, connect the positive/hot signals. If you have any point upstream that you want the signal to remain stereo, you'll need to use a buffer stage. As a refinement, you can use a resistor array to blend the signals but it's not, strictly speaking, necessary. Don't try to wire the outputs of an amplifier together, you're likely to toast the amp. Low level signals can be added with impunity.

If the two channels of a stereo signal are combined together by a direct short, there will be a drastic drop in the dynamic characteristics of the signal and a significant loss of high frequencies.

The best method would be to add them resistively followed by a buffer (preferably in the inverting mode). If needed you may need to invert the signal again to maintain absolute phase. Follow the schematic of a summing amplifier from any standard text book and you should be done.