Kevin, there is not a fix-it-all formula in the years ahead of you. I know you will try to look for one. In that process you will try to read about methodologies and the lives of physicists who were where you are right now. Indeed, nothing will account for the learning from your own future experiences. Here I am leaving you some words for you to take in your steps forward. First, the length scales in a PhD are going to be very different than in undergraduate. And second, you will have narrow much more your focus.
Every new PhD student has to face the difficult decision of choosing an adviser. During the next 5 to 6 years, the student will work under the guidance of the adviser, with efforts of trying to learn the craft of doing scientific research. Those were the thoughts in my mind back in the Fall of 2015 as I started fresh in the ELE department at Princeton University. Now, as a senior PhD candidate, I have earned new insights along the way that are worth sharing. Mainly, I want to tell you that you do not only join a PhD adviser, but a whole team of people working around a similar goal. Looking back, I want to write down that the process of choosing an adviser is better described and tackled as choosing a laboratory where to work on.