Define your problem and requirements: figure out precisely what you want to build and why. It’s okay if this changes over time, but you need to start with some concrete idea. Writing this out is a good idea.

Make a high-level plan: determine what the components of your system will be like and how they’ll fit together. Each component should play a narrow, well-defined role. Working through this on paper is probably a good idea as well.

Pick your platform: decide what hardware and operating system you wish to target and what programming language and libraries will best serve your goals (that you either already know well enough to use or are willing to learn along the way). You may have to target operating system-specific APIs. Try and determine what they’ll be.

Initialize your project: using the package manager and build system associated with your language and libraries, initialize a new project. There should be functionality to integrate/link libraries, build/run the project, and run tests. You should also version-control the directory.

Start prototyping components and writing tests, all while committing these changes: build things according to the role you expect them to play in your system. Write tests to ensure they behave as expected, on trivial and non-trivial outputs, ideally exercising as much code coverage as possible. Review your code to make sure it’s basically comprehensible, upholds the invariants you expect, and handles edge cases correctly. Whenever you complete a small change or enhancement, commit it.

Get something going: if you have to do a lot of experimenting, try and get something basic working, then step through the changes you’ve introduced in the version control. If you have a big, risky change or want to develop some new functionality that’s largely independent of other functionality, start a new branch and merge it on once you can vet it.

Integrate and refine: once you have components that you’ve tested, write code to integrate them and test cases for your integration. By this point, you should have something resembling your envisioned whole program working.

Pay down debt: once you have a new feature or behavior working, try to pay off technical debt by refactoring your code to make it easier to work with. Fill your source code with documentation.