There are many ways to get a good sub bass through external FL Studio plugins but this is a way to get one through a built in plugin, the 3x Osc.
1. Go to 'Channels' and hover over 'Add One', select '3x Osc'.
2. Make sure you have 'Typing Keyboard to Piano' turned on. (Options > Switches > Typing Keyboard to Piano).
3. In the 3x Osc plugin, turn Osc 3 volume down to 0.
4. Push Z, X & C. These are your C5, D5 and E5 keys. This is just to confirm what kind of sound you are getting.
5. Turn the CRS dial on Osc 1 all the way down.
6. Turn the CRS dial on Osc 2 all the way down.
7. Push your keys, you should hear a sub bass sound.
8. Change the pitch using the little keyboard underneath, right click above the keys C4, C5, C6 etc to change it.
For sub drops:
1. Enter the Step Sequencer (shortcut F6)
2. Right click on the 3x Osc and click 'Piano Roll'
3. Enter a length of the note you want, to test try just stretching to one bar.
4. On the top left of the Piano Roll, look for the little triangular Slide symbol next to the light green rectangle.
5. Stretch the Slide note to any key you like, this enables a pitch up or a pitch down.