No worries mate, it's a common mistake.
I think you're confusing mastering with mixdown, which is also another common mistake. Mastering is done post production, usually outside of your software, and is usually better done by a sound engineer at a proper mastering studio.
Mixdown however is what the artist does throughout his / her track to make sure all the sounds sit correctly in the mix, and all volume levels and stuff sound right. As for tips on how to do this, it's really difficult to explain. It takes alot of practice and time to get it right, I still have trouble sometimes with this and I've been producing for nearly 3 years now.
There's no set way to do it in FL than any other DAW, it's all the same. All I can suggest is watching tutorials on youtube, chatting to some of the guys around this forum (This is my best advice, as it's how I learned most of my production techniques) and trial and error. As a basis, I would suggest to always mono your bass channels, along with your kicks. Cut anything below around 90hz on the kicks, and maybe give a subtle boost around the 100 - 150hz range on them. Snares can sit slightly mono'ed, or centre. I usually layer a couple of snares, boosting one of them around the 300hz level, and the other around the 600-800hz range. Hats, and higher range sounds can be panned slightly to the left and right, or stereo seperated to give a sense of space. Any FX you may have you can go crazy with delays and reverbs etc, panned either direction etc etc.
This is just some of the things I do, and to be honest, others could come here and say thay are wrong too.....it's just what works for me. I'm still learning new stuff each day. Like I said, my best advice is to hang around this forum, there's a great community here, most of which are more than happy to help out. So, here, tutorials on youtube, and lots and lots of trial and error......its the way forward!