Writing code
Writing code is the skill most associated with software development. It is no doubt the most visible result of your work and the thing that most people associate with contributing to open source. This makes many developers imagine that to practice this skill, you need to write code from scratch. That can work, but it is not the best way. You also need to write code that you can compare with other code.
Refactor code
To really use open source to improve your code-writing skills, you can refactor code. You first use refactoring to understand code better. By breaking down its parts and understanding what each part is doing, you will have a clear vision. You can then try to refactor the code to make it more readable, more efficient, or cleaner. When doing these exercises, you probably will not even contribute the changes back to the projects and just use them to improve your skills. This will help you contribute to the project in the future, since you will better understand...