The easiest way to master in fl studio is to put the Fruity Multiband Compressor on the master channel, put the limiter on, adjust the volume gain underneath the limiter button if you need to. Then fiddle with the low band/mid band/high band gains. Start at about: low band 3.0db, mid band 4.9db, high band 4.5 then mess about and see what sounds right for you. If you're new to mastering this is a good way to start.