If it was arranged as above in vince's post, then your directions would be something like:
have a variable along the lines of 'currentPosition' which will be the address of the LED for your function leds[currentPosition] = CRGB::White or whatever.
if up button is pressed, currentPosition gets +1
if down button is pressed, currentPosition gets -1
if right button is pressed, currentPosition gets +6
if left button is pressed, currentPosition gets -6
then you can decide if you want wraparound or just boundaries, so something like
if left button is pushed and currentPosition = 3, do nothing
So, take your time, try a little, and logic it out.