Pro tip for anyone having trouble with this. You have to use multiple rounds of heat and pulling. Use a hair dryer and not the iOpener that comes with the fix kit and get it just too hot to touch then quickly start pulling and wiggling it up and down. Once it cools down grab the hair dryer again and repeat it a couple times until you start to get a big enough gap.

I just did this yesterday and it took around 5 rounds of heating and pulling to get it to seperate enough from the body.

Correct me if I am wrong but you no longer need to do this as of IOS 15.2 as apple doesnt disable Face ID on 3rd party repairs