How to start designing a desktop application

In no specific order

• Write a plan out for what the application is to do rather than just code
• Write unit test which can assist when something does not work as expected along with pointing to code that may fail that can indicate the need for try-catch statements which works in tangent with the next bullet.
• Implement logging, consider SeriLog.
• Stay away from writing all code in a form, strive to place database operations in a class, same goes for working with files
• Consider using classes over DataSet and DataTable and stay away from TableAdapter
• For data operations either Entity Framework Core or Dapper
• Decide on either MAUI or conventional Windows Forms
• Organize code in folders e.g. for classes that perform work in Classes folder, for classes that are containers for information create a folder named Models etc
• Create a readme.md file which provides high level information about the project and points to more detailed documentation.
• Create a private GitHub repository for the project

And consider a web solution over desktop.