There are two different ways to go about this. If your floor joists are only off by 1/4-inch to 1/2-inch, you can install the top hangers directly to the floor joists as stated in the installation instructions. When you have all the top hangers and cross tees installed, you can then go over the grid and back out the screws in the top hangers to adjust for the uneven floor joists. If your joists are off by more than 1/2-inch, you will need to shim your joists so that they are level. Then you can begin installing the top hangers and cross tees.