Sure you can use FL Studio for mixing. And arguably it's considerably more capable than a beatbox, or a hardware mixer, but that entirely depends on what you're using to do the recording/arranging. It's probably the skill of the user that would most accurately determine how well the song is mixed, regardless of what they used to do it.
You can also use, Reaper, Cubase, Sonar, Nuendo, Studio One, Ableton Live, Logic, Mixcraft, Pro Tools, Samplitude, Sequioa, and any number of other DAWs for mixing so I'm glad you cleared that up.
There are thousands of tutorials out there for FL Studio also, so it might be a lot easier on you if you just try to collect a bunch of the ones that have already been done. That way you only have to write the ones that you can't find any information on. But a quick Youtube search will return literally thousands of videos on how to use FL Studio for any number of things.
It seems like you're posting this in response to something someone said?