First you need to find out the BPM of the original song. Use like virtual dj or something.pen
Open up FL studio, and right click on one of the channels in the pattern and go to insert -> 3x OSC
In the channel settings of the 3x OSC, turn the volume knobs for OSC 2 and OSC 3 all the way to the left. This should give you a just a tone.
Set the BPM of fl studio to the bpm of your original song. Open up the piano roll and make your bassline how you want it. Depending on how you want it, this could take multiple patterns and might not be very easy.
After that go to the mixer, set the master volume to -3 db then export
That's the way to make the bassline. You can use audacity to put the two together.