Do you go ahead when you’re ready or go ahead when you have an idea?

I see these two advices float around the internet a lot. They’re both very popular. The former describes that you must have a solid plan and foundation for the idea you want to embark on, if not it’ll fall apart before you even begin. The latter describes that the moment you have an idea, you ought to act on it because if you don’t, someone else will, and also because the future is unpredictable so there’s no point in planning every single step. They are both great advice, but they conflict. How can we utilize both?