Hi, I had my assessment with Nail Harmony in February and passed so I know how you feel! I was so nervous! Definitely take your time, and arrange all your products so you can see them easily and won't feel flustered trying to find something. Take a model you feel comfortable with if you can.
We were told which nail to do at a time, we started with clear, then pink and white and then the extended nail bed pink and white, I was one of the last to finish each one but felt pretty happy with each one and some of the quicker people didn't pass as their filing was too rushed and unfinished so do take your time to perfect the nail.
Oh! Make sure your files are sharp too, and you've got a brand new green and white buffer as it makes it so much quicker and easier to get that shine!
If one goes completely wrong and you can't salvage it without soaking off, leave it and start again on a fresh nail, my first extended nail bed was awful as that was the one I was most nervous about, so don't be afraid to scrap it and start again!
My assessor was sat at her own table, she didn't hover over us so you won't feel like you're being watched.
Hope this helps!
Lucie x