i have a completely black glossy guitar.

i wanna put a small design (like smiley face or something like that) on it, or maybe write something on it.

what is the easiest and the best way to do it?

I like guitar...
Sand paper, sharpie, and clear coat.
In that order. Spending time and doing it right will pay off though.
A silver sharpie alone will work fine, I drew a little cartoon guy based off of myself on an epiphone les paul jr. I used to have. It looked pretty good and didn't rub off, just make sure you don't make any mistakes.
get a pinstriping brush (10 $) and some acrylic paint and voila!
just make sure to practice on something else to get used to the brush
GUITAR: Gibson Les Paul Special
AMP:Fender Vibro Champ XD