This is what I do- I used to have a fear of putting pen to paper
Ask for scrap paper before the exam starts- you can't take in your own paper.
I immediately write my name on the scrap paper- I know that I remember my name I can put it on this paper and commit.
I through the questions , jot down on the scrap paper any ideas I have-however random, maybe nothing to do with the subject a bit like my EF posts. Go back to the exam paper read it again. jot down more ideas on the scrap.
Glance through the scrap, start to put the ideas into some kind of structure, other things come to mind, write them down. I don't look at other people, stay in my thought bubble, don't get distracted.
Go back to the exam paper- start with the question you feel most comfortable with, you probably have something- start writing. If more answers randomly pop into my head I jot them on the scrap, I can organize them after you finish the thought you are dealing with.
At the end I re-read my exam answers. Check my scrap paper, did I miss anything?
You can model it at home I did it with my kids - get them to close their eyes- imagine lining up for the exam to go into the exam room, being in the last exam, get them to talk about how they felt, feeling the tension in their neck and hands, feeling sick, heart racing whatever. Then tell them that you are going to give them the last exam paper they had, without their answers of course. Get them to redo it, but give them as much time as they like. Reward for effort- saying nothing about what they got right or wrong.
Start to give other exam questions go through imagining the situation again and start to restrict the time, then very gradually introduce critiquing and grading.
Until you get to the point where they are under a lot of pressure with time constraints, with no notice with the clock ticking away beside them.