First of all, where am I going? And why?
Then the route and method of travel. After that, I can decide if I need to make reservations or just wing it as a I travel.
And, a crucial step is being aware of weather and dress needs so I can pack efficiently. No need to take a parka and a toque to Chicago in August.
Ten years ago, I went to Italy for twelve days with a general idea of where I would be going, but without firm plans. That gave me the freedom to spend extra time in Cinque Terre and in Pisa, both of which I loved, and not so much in Siena, which was a medieval walled tourist trap.
But I am taking a long holiday weekend, and have my accommodations and rental car all reserved. Since I am going to a music festival, my schedule for most days is pretty well set.