In order to answer your question specifically...first start by digging up quality samples...that is where you start first...fl studios is only an extension and will only put out what you put...it wont make a shitty sample sound better.....
Start digging thru these sites listed here
https://www.illmuzik.com/forums/showthread.php?t=360
Next you'll wanna looking into and understanding soundfonts... this is usually a cheap and free way to get an instrument, we all use it here...
http://www.sf2midi.com
You add these thru the fl soundfont player you will need winzip to unpack them
After understanding and finding a good bass soundfont instrument look into studying several methods of increasing bass by experimenting with eq in fl or layering two or more instruments with the same notes what i usually do is get two bass soundfonts one lower on the octave which will give the rumble the movement of the sub....which are the notes we dont hear but feel (you need great speaker to test this) next copy the notes to another soundfont higher in the octave this will give that hum...vibrating sound that we can hear..channel both sounds to a free fx channel..add compressor and eq..tinker til it pleases you...its always subtle to blend a dry kick in there too...depends on the genre r and b you leave off the kick... hip hop add it to give a thump and definition..