Please watch the video below for a quick overview of the process. The key things to remember are to not cut the rubber loop, you just need to work it around until any slack is gone. Also remember to ensure it's pressed in well all the way around, to prevent it from coming loose.
Tip: If you find the kit is loose in one or more of the channels, this is because the alignment of the roof glass can vary quite a bit across the Model 3 fleet. If this is the case, we recommend a small (1"/30mm) dab of silicone sealant in the middle of each edge, as that will hold the rubber in place, and, should you choose to remove it later, is completely reversible, as the sealant only makes a semi-permanent bond to the rubber seal on the car.
Or watch another great video by Erik: