Adding a New Feature

(I'm new to this community, but I thought I'd dive right in.)

I love to code but when you have a project that spans many functions across multiple files, I have a hard time isolating what I need to code up a new feature.

The way I do it now is to make a new file, determine the input I'm getting and the output I want.

Once I have the code that I want I cut/paste the function back into into my codebase.

I want to build something that will automate this process. Maybe an extension for a code editor??

I'm not sure where to even start. So I'm asking for help, or even just suggestions.

Thanks!