I found this sticker on Behance by Manuel Cetina. The limited colors and design really drew me in to take a look at it.
One design skill that is very impressive in this in the halftones. He uses no shading, only small dots and filled shapes to give the skull depth. The sunglasses especially have great halftones.
The second thing that makes this look professional is the line width. One mistake many beginners make is using small, black lines around their work. This artist used thick, black lines to outline his skull.
The decision to change the perspective of the skill was crucial here. Imagine it being straight on. It may still look great, but the change of perspective makes his sunglasses stand out, and we can see the details on the side of his skull.