Icon design is a top-tier skill comparable to calculus: there are prerequisites for both that you must learn beforehand. For math these include operators, variables, and deductive reasoning. For icon design you need to learn color, shape, and form to eventually create quality work.
Line icons have bridged the gap to make practicing & learning a tad bit easier. Designers can practice line icons without needing a lot of detail in shading or color. If the proportions look good and the shape is recognizable then it’s a useable icon.
This gallery offers a sample of how you might start practicing to improve your own skills. Learn from these iconsets by following the thin line style and see how you might implement a similar technique in your own workflow.