Export the finished song as a WAV file. The volume should be somewhere between -6.0 db and -3.0 db to leave some headroom (a safety buffer calculated in decibels between the highest audio peak signal being produced and the maximum that a piece of equipment can handle).
Create a new project in FL Studio. Click on "File" then "New." Erase all of the unnecessary silence at the beginning and end of the song. If the song calls for it, you can also create a fade at the end by highlighting the appropriate section of the song, clicking the "Tools" icon (looks like a double-ended wrench), then "Fade Out." Alternately, you can use the shortcut "Alt-F."
Nomalize the WAV file. Click "Tools," then "Normalize" or "Alt-N." This raises all of the audio peaks by the same ratio with the highest peaks being at 0.0db preventing clipping and distortion. Create another empty project file. Import the WAV file to a playlist.
Load the mastering plugins. This is a suite of virtual streaming technolog plugins that come with FL Studio and include Fruity Limiter, Fruity Compressor, Fruity Multi-Band Compressor and Fruity Parametric EQ 1 and 2. They can be accessed by clicking the "View Mixer" icon at the top (it looks like two meters side by side), Then click the down arrow on the "In" master bank on the right side of the mixer. Keep in mind the order that they are entered will determine the overall sound. Generally, put EQ before compression.
EQ the song, listening first for discrepancies in the lower frequencies then working up to high frequencies. Listening through to the end of the song will give you an idea of what needs to be fixed, and you can begin to isolate each frequency as needed. Use soft compression so there's no distortion or muffling. Export the song when finished as a master WAV file.