The tracker attachment has an opening of 25mm x 3mm at either end for sliding the collar and buckles through. Since it's silicone, it can stretch to accommodate a slightly larger or thicker collar.
For Cat's, the tracker is best suited on medium to larger sized cats.
This video may help.
Please note that the LED Night Collar is intended to wear alongside/over the top of your pet's regular collar when in dark settings. Do not use the tracking device on the LED Collar, please use your regular collar.