[Phanx]

Really it depends on the actual addons in question, but in general I'd say you should:

1. Fix the existing addon for yourself first.

2. Then, once you have a reasonable idea how WoW addons work, write your own addon. Start with the most basic feature(s) and hardcode any options, then add more complex features as you go. You can add options later.

As for reusing code, again, that depends on the specific context.

It's probably fine to copy a function that scans a tooltip for text, for example -- there's really only one way to do that, so even if you write it from scratch without looking at any preexisting code, it's still going to be the same code, just with different formatting and variable names.

But it's not fine to copy the whole addon and just change the variable names and some formatting details, and in my experience, with a lot of addons, the overall program design and coding practices are so bad it's just better -- and actually easier and faster -- to just rewrite it yourself.