As you know, existing projects would typically have maintenance/enhancement type of work, below steps will help you guide your job as a developer in the easiest and most efficient manner.
- Get a complete clarity of the requirement/enhancement from product owner.
- Understand the whole flow that require a change starting from origin of request, till the output ( how and in what form)
- Do impact analysis, think about all the channels or sources from where the request will arrive, what components will be impacted, any reports impacted all of it.
- Estimate the story with points based on the above impact.
- If development involves a design decision, always have discussions with senior developer/tech lead and get your design reviewed before starting any implementation to avoid loads of rework. Also you get bonus points to build that trust with them.
- When coding is complete, make sure to do a self-code review.
- JUnits should be covered with all positive and major negative scenarios.
- Functional testing of your own code is must and should cover all scenarios, before deploying code making it available for testers to test. Think in a way testers would test your story will help you visualize more relevant scenarios and catch any issues beforehand.
This way the bugs will be minimal or none, and soon you will be get a grip of domain and process at hand.