advice for exposure therapy (imaginal exposure) in tf-cbt?

i've done training in imaginal exposure in tf-cbt and i understand somewhat where to start and end the imaginal exposure, but i wondered if anyone had any tips/advice for confidently assessing the right points to start and end the exercise?

and also, in my training they talked about recording the imaginal exposure so that the client can continue on for homework but i was just wondering if anyone had more advice on this area? i don't quite understand how it's meant to be done for homework?