VST Connect pro with Cubase 10

Hello. I couldn’t find any good tutorial. Yesterday, I tried recording with a friend and it worked fine but I’m not sure of how to set things up so I can tweak his headphone mix. I think it’s with VST connect cur send but sometimes he had no sound and sometimes he had sound and I didn’t really understand why. We just had a few minutes to try but we will take more time soon to experiment.