This is the first decent drawing of inkmatas I could find
Anyway
If you look closely at her hair/tentacles you'll see that they're bigger at the ends, I always do this to stay at least semi accurate to the game, luckily it's a pretty easy detail to add
(Note that I draw her hair longer than usual so she looks more grown up)
Another detail you could add that I didn't in this drawing is making the ends darker at the tips, and having that kind of fade up into the rest of the hair, another way to make it game accurate
As for the dark part around the eyes I generally just make two circles around where the eyes will be and then connect them with curved lines, then I make the eyes in white
That's pretty much everything I learned from drawing inklings specifically, octolings are far more difficult though.