One thing you can do for matching Hearthstone's style and level of execution is to check out the personal pages of your favorite card artists. A lot of HS artists show step-by-step guides on their artstation, DeviantArt, or personal pages for their card paintings.
It can also help to disassemble your favorite card art. Take it into your editor of choice and isolate just the values, just the colors, posterize it to it's core white vs black value scheme. Zoom way out until you can barely tell what it is and redraw just that little cluster of shapes. Smash it into tiny pieces and put it back together. Btw, value control seems to stand above the other art fundamentals for making great card art, lol.