I do not know your FFT library but it should give you a table of frequencies and amplitude.
Then you have to group your frequencies. You have 8 columns of leds, so it would be something like
20-50;50-100;100-200;200-400;400-800;800-1600;1600-3200;3200-6400 Hz (8 bands)
Then you choose what to do with the amplitude data from these band (you can try and take the max value or avg)
Then you divide each amplitude/band data by a number so that it is easier to route it to your leds.
Basically you want for each band to have a number between 0 and 5 so that you know the number of leds to power.