Each car has a different key cut for it.
The transponder in the head is unique to each car,without the right combo the car cannot start.
That is why you need the key cut to your existing lock combination then the head needs to be programmed to your car.
If it is just a shell that is broken but you already have a key and transponder from the damaged one then it is real easy,.
I did one last night for a friend where i took one of my extra shells with someone elses key blank ,.
I dremeled the plastic retainer off the top of where the key is sealed in the shell head, Then slid it up ,so when he got home he just needed to slide his key into the shell i gave him ,insert the transponder which he alreayd had .Screw it together and he would be done. No programming or cutting needed since he alreayd had it from his broken key,.