I find it really depends on the individual. I know people who can just jump into a project / task without a plan and able to complete it very well. I also know people who can't even begin to work without planning out every little detail.

Personally, I like to have an idea / an outline of the project and try to divide it up into smaller tasks that I complete according to dependencies and according to urgency. At the same time, I always keep in mind that things don't go according to plan most of the time, and will change my plan / adapt along the way, according to the current situation.

I find that having a plan that is simple, straight forward, and easy to communicate is especially important when working with other people. Other people cannot read our minds, at least not yet, so it's better to have a plan that can be communicated to others such that others know what you will do and what to expect. As misunderstandings and arguments usually arise from miscommunication and difference in expectations.