How to set LR & mono outputs separate?

I bought new QU16 desk last week, and I found out that it’s a little bit different with X32 consoles. My question is, how to set separate LR and Mono outputs. Let’s say, if I want to send Low frequency instrument to Sub/mono channel, can someone explain how to do that? Thanks in advance.

You need to use one of the mix/aux outputs.
Set that to post fade, post all mode.