Well, it's got 8 main line-outs, so it should just be a case of 8 TRS to TRS cables, though an 8-way loom may be more convenient. And then you'll want your outputs 3-10 connected back to the desk if you want to use it as a proper analogue mixer.

But the mic inputs on your Focusrite are probably better than the ones on the desk, which will invariably add more noise.

Adding a mixer to a digital recording setup isn't normally a wise move unless you really want to mix in an analogue way. But then you really need a fair bit of analogue hardware like compressors and reverb and gates etc. otherwise you are just using it as an analogue summing box.