The X mark on the forehead represents a throbbing vein. It originates in manga and became kind of widespread across all east asian and east asian inspired comics.
Eyes going all white when angry is to symbolise that they've become "like a monster" they have so much rage.
The best way to learn this kind of stylised shorthand is to read lots of comics, draw lots of your own and to develop your own stylised ways to get across funny exaggerated emotions. Some of mine are influenced by the manga "Azumanga Daioh" for instance, and I also use dot eyes sometimes to show a surprised or confused, blank expression, which I think I picked up from "Scott Pilgrim". Different comics do it differently, so see what works for you!