How do you develop emacs core packages?

[Ackerley Tng <ackerleytng@gmail.com>]

Hi Emacs Developers!

Would like to learn your workflow while developing core packages.

I use the package manager's version of emacs, so I have a lot of
.el.gz files in some system directory.

If I want to develop on one of those, I usually make a copy of the
entire file, and then iterate by evaluating the function in emacs to
override existing definitions, but sometimes when global variables
initialized on emacs startup need to be adjusted, I find it hard to
undo those steps, which the default package sets up.

What's your workflow for developing core packages, do you have any
tricks to share?