Listen.
Any time you're stretched beyond what you think you can do, you're going to feel nerves. You got the assignment for a reason. Kill it.
I'd focus on the following... (1) do you understand the play clock rules, (2) is there a communication path that will help you fix small mistakes, and (3) do you know how to reset the clock.
If you have all three, go work the game confidently. If you make a mistake learn from it and don't make it again.
All three are valid points and your last point is very very true.
As far as point number (2) goes, If I am in doubt, set the play clock to 40 seconds and let the R tell you if it needs to be set to 25 (one open hand pumping up and down).
As far as point (3), I always make it a habit to get to the school/stadium early, so I can have a quick pre game chat with the R as to how he likes to run the game, then I head up to the pressbox, so I can learn how to operate the game/play clock, especially if there is no one up there to show me (But I am usually good at figuring things out for myself and if I can't figure it out, I will find someone to help me).