Get to know the brief and start building your calculators from scratch while recapping important object-oriented and software engineering principles such as the Don't Repeat Yourself (DRY) principle.
The basics are done, and now it's time to spruce them up! Continue with adding documentation and automated testing, and extend the functionality even further.
The main code is written so now it's time to make sure everything works as it should and follows the best practices. Learn how to review, refactor, and improve your code, as well as run more comprehensive tests.