The way that I ensure my code is functional and secure is by following best practices and always testing. I run tests from the first line of code, all the way until the last line of code. This ensures that my code is always functional and secure.
When it comes to taking user needs and requests, I'll start by taking a piece of paper and writing how each "need" should work. I then write out how the code works in pseudo code, and then translate it to real functional code. This helps ensure I understand how each function should work and it gives me something that is easy to translate into code, speeding the coding process up.
I start by understanding how the system should work, and then break the system down into small pieces. I then create the first small piece of the software in a way that is modular and reusable. By creating modular and reusable pieces this allows me to use and adapt the code to other parts of the system. Creating less bugs and less syntax mistakes. I will also adapt the modular code to reflect the needs of that part of the system.