Implementing design principles and best practices on old code can be a challenging task, but it can be done effectively with the right approach. Here are some steps you can follow:

1. Assess the Current Codebase: Start by evaluating the current codebase and identifying areas that can be improved. Consider factors such as code complexity, maintainability, and the risk of introducing technical debt.
2. Prioritize Refactoring Efforts: Focus on the most critical parts of the codebase first and prioritize the areas that will have the greatest impact on reducing technical debt.
3. Plan the Refactoring Process: Plan the refactoring process carefully and consider factors such as the dependencies between different parts of the code, the impact of the changes on existing functionality, and the potential risks of introducing bugs.
4. Use Automated Tools: Automated tools can help to streamline the refactoring process and reduce the risk of introducing bugs. Consider using tools such as code linters, static analysis tools, and automated testing frameworks.
5. Adopt Incremental Changes: Make incremental changes to the codebase, testing each change thoroughly before moving on to the next. This will help to minimize the risk of introducing bugs and ensure that any issues can be quickly identified and addressed.
6. Document Changes: Document the changes made to the codebase and update the code documentation accordingly. This will help to ensure that the code remains maintainable and that other developers can understand the changes.
7. Continuously Refactor: Continuously refactor the codebase over time to ensure that it remains modular, maintainable, and free of technical debt.

By following these steps, you can effectively implement design principles and best practices on old code and reduce the risk of introducing technical debt.

How to apply this to code that is built on a monolith parts

Breaking up a monolithic codebase into smaller, more manageable parts using SOLID principles and the Model-View-Controller (MVC) pattern can be a complex task, but it is essential for reducing technical debt and improving the maintainability of the code. Here are some steps to follow:

1. Evaluate the Monolithic Codebase: Start by evaluating the existing codebase and identifying areas that can be refactored. Consider factors such as code complexity, the amount of duplication, and the risk of introducing technical debt.
2. Implement SOLID Principles: Adopt the SOLID principles to improve the structure and organization of the code. This will help to make the code more modular, flexible, and maintainable.
3. Apply the MVC Pattern: Use the MVC pattern to break down the monolithic codebase into smaller, more manageable parts. This will help to separate the different concerns of the application, such as the data model, the user interface, and the logic.
4. Refactor the Code: Refactor the code to apply the SOLID principles and the MVC pattern. Consider factors such as the dependencies between different parts of the code, the impact of the changes on existing functionality, and the potential risks of introducing bugs.
5. Adopt Incremental Changes: Make incremental changes to the codebase, testing each change thoroughly before moving on to the next. This will help to minimize the risk of introducing bugs and ensure that any issues can be quickly identified and addressed.
6. Document Changes: Document the changes made to the codebase and update the code documentation accordingly. This will help to ensure that the code remains maintainable and that other developers can understand the changes.
7. Continuously Refactor: Continuously refactor the codebase over time to ensure that it remains modular, maintainable, and free of technical debt.

By following these steps, you can effectively break up a monolithic codebase into smaller parts using SOLID principles and the MVC pattern, reducing the risk of introducing technical debt and improving the maintainability of the code.