...in FL studio, that's pretty simple. I'm guessing you're talking about a note that is played and (on a real bass) the bassist slides his fingers on the cord0 to change the pitch of the note he played... and usually, they go to a lower note...
My suggestion? if you have a program that accepts samples, find a good guitar sample with a good pluck and a sustain option (like loop point, for instance) or a good guitar soundfont, if you can have one...
normally, there is an option in FL Studio' PianoRoll that is a slide (and I'm pretty sure it exists other programs too)
no5 on the picture below... The lenght of the rectangle represents the time it will take the note to go from where it is to where it will be at the end. The shorter the rectangle, the faster the slide will be.