How do you decide to implement design pattern and principles in a project?

Let’s say you are want to build a web application and you have the requirements as well. You have done an analysis on high level system design and requirements. Now when it comes to setup your project with respective stack, how should one think of organising or structuring the project with the help of principals, design patterns, etc. So that the code is scalable, modular and also make other upcoming developers life easy. Not looking for any specific answers but anyway having their experience can share the thought process and decisions they have taken while doing something similar.