Start FL Studio. If the original settings haven't changed, the step sequencer and mixer will automatically load. Adjust the tempo and the pattern length of the song as desired.
Find the bass sound you want in the Browser folders on the left hand side. FL Studio comes packed with many sound samples which can be identified by a small orange icon with a black audio waveform. Left-click on the name to preview the sound.
Load a bass sound from the browser into the step sequencer by either clicking on the sound and dragging it to the step sequencer or right-clicking and choosing, "Open in a new channel."
Select the bass channel if it's not already selected by clicking on the oval channel light next to the name or the name itself.
Open the Keyboard Editor. This is the piano keys icon at the top right corner of the step sequencer. A window will open that looks like multiple keyboards lined up vertically. By clicking on the keys, you can hear the bass notes.
Press "Play" to hear the sequence. Experiment with different bass sequences by clicking on different notes and steps. When done, click "Keyboard Editor" to hide the keyboards.
Right-click the bass channel and choose "Piano roll." The channel steps will change to a bar grid which will give more precise control over note lengths.
Click on the bar grid and a magnified window will appear.
Hover the cursor over the green bars. Hovering near the center will change it to a four point arrow, allowing you to drag the bar vertically. Hovering near the left or right edges will change the cursor to a two point arrow, allowing you to adjust the bar's length.
Press "Play" to hear the sequence.