start > programs > accessories > entertainment > volume control
options > properties
hit the recording ratio, and select whatever input you're using. hit ok
select whatever input you're using, and put the volume up maybe 80%.
in flstudio, select an open insert, where it says 'IN' at the top of the mixer, choose one of the options (all my options detect sound, so choose whether or not you want mono or stereo (usually you want mono)).
if you speak or play something, it should run through fl. turn down the buffering if you don't want much/any latency.