Load the program and the song with the guitar track that needs editing. If you have an older version of FL studio, you can still change the way guitars sound, but you will have limitations on what you can do.
Open an equalizer as a plug-in for your guitar's track as you normally would for your version of FL Studio. If you have any third-party plug-ins or a version that is 10.0 or later, use a parametric EQ, preferably 7-band.
Select any of the mid-range bands, avoiding the extreme high and low bands, and make this band narrow. Boost the gain, or volume, of that particular frequency as much as you can. Play your guitar track and begin to slide the band left or right to boost a narrow range of frequencies. When the unwanted sound becomes extremely loud, and perhaps, piercing, you have found one of the bad frequencies. Lower the gain to around -6 db (decibels) or as low as you need to eliminate that sound.
Continue to locate and lower other frequencies that are undesirable. Bypass the equalizer or click on the "Compare" button if you wish to hear the difference between your "eq'd" guitar and original guitar.
Use the "Slice" tool to click on and cut out space in between playing segments where white noise, accidental notes and other unwanted sounds may occur.
Add effects like delay and distortion to mask small errors in timing and missed notes.
Re-record your guitar if you or the person playing did a poor job. There is no way to better or perfect a very poorly played track. This means you must also use good equipment, such as new strings, cables that do not buzz and equipment that is not too old or broken.