I have a Yamaha MG10XU that I'm using with a DDJ FLX6. I'm using 3 ft RCA to RCA to connect

I tried to find the answer on youtube but couldn't get any results. I'm confused about where to place the gain at the bottom of the channel in the mixer, and the master volume (the red knob on the bottom right hand side). Should I turn up the master before turning up the gain on the channel? Or should I turn the gain where I want it and then turn the master knob to get the loudness that I want in the venue?

I know that on my FLX6 I should keep the trim levels out of the red but I'm also confused on if I should turn my master volume on the FLX6 all the way up before it's sent to the mixer.

Any advice is appreciated.