I'll be starting a science PhD program in the fall, and I'm required to do at least three lab rotations. I haven't received any instructions from the program as to when and how I should secure a spot in the labs I want to rotate in. I did contact some of the PI's I'm interested in working for while I was applying, as well as during interviews, but obviously nothing was finalized. Should I be contacting them and trying to meet with them now, or is this generally done at the beginning of the school year?
Have you even shown up to orientation yet? I think you're getting ahead of yourself. But if you're already in the department wandering around lost then my advice is:
It obviously varies by department but there should be some sort of form you fill out with your top picks. You have a few options: contact your graduate advisor and ask; go talk with a perspective PI and bring it up during your conversation; or ask the other students in your program if they know what to do. The third choice is by far the best. It seems like you're just sitting there in a vacuum wondering what to do. You will quickly die if you do that. You need to meet the other students and you need to work with them to make it through.