Pick a good IDE, set it to the most pedantic mode you can, and work on the code until there are no errors, no warnings, and only acceptable info notifications.

Pick a good IDE, set it to the most pedantic mode you can, and work on the code until there are no errors, no warnings, and only acceptable info notifications. Have a good test suite with realistic scenarios. Add more to that all of the time. Get outside feedback on whether you documentation and diagrams are readable and keep them up to date. If you can do that, you can write mission critical code.