I would recommend employing the Agile approach to development. Create and test a wholly working prototype of whatever you're working on, be it a new feature or

I would recommend employing the Agile approach to development. Create and test a wholly working prototype of whatever you're working on, be it a new feature or extending a function. The key is that the prototype should not by any means be the final product, instead aim to create something that is working and accurate to a small step of the problem; an iterative approach to development. This way, your tests grow with the functionality of your code and if your code is well tested, it should function properly at every step of the way. Thus, by the end, you have a fully implemented whatever that has solid tests and should "Just Work". If this seems too slow or tedious, you can take the move fast and break things approach, in which you simply create a rough prototype of how you think the code should look, and work through the bugs until it does what you want. You should still write tests, especially to help determine when you've properly fixed the implementation. Keep in mind this involves a stronger sense of underlying problems in bugs and needs good debugging skills to be effective. It can take more time though if your initial prototype is too far off.