1. Try to see what existing functionalities are there and how exactly the code is being executed line by line by putting debug points on the codebase.
2. Try to change the local codebase and see how the changes are affecting the application.
3. Whenever stuck - try your best to find out the errors and search for the solution for the errors present on the console. If facing more difficulty, better take help from seniors.
4. Think of the simplest approach you can use to develop a piece or make a change without affecting much of the codebase.
5. See other projects which uses same technology but with small codebases having same kind of implementation and try to learn the best practices followed also do visit documentation regularly because sometimes all you need is already explained in the simplest way.