Your collar will come with 3 sizes of velcro straps.
There are no hard and fast rules for putting these on. If you think your cat is small, then start with the smallest one. Our Monty is about 5kg and is currently on a medium. Ideally the square GPS housing should be at the back of the cat's neck as this will offer the best position for tracking.
It can be a bit fiddly to remove the velcro (Callum in the video is a bit of an expert!) but if you gently pull other side of the plastic clasp aside it should easily come off. You should have 3 plastic tabs, so it is not the end of the world if you lose one.