How do you implement new feature/functionality in your project?

I started my career as a Java developer half year ago in an existing project. When I start implement new feature I just look for similar feature in the project and copy it with slight modifications. First time I was happy with this because you get task-you do task and it works. But now I don't feel like I get much valuable experience. I just copy others' code. Other teammates say they all do the same))

So, what is the proper approach of doing this? Which books describe this? I heard about SDLC, but how to implement this? Please, help