That said, I was wondering whether the experienced contributers here
might offer some advice for the day of my check-ride. My main concern
is that I might be so nervous that I bungle the easy stuff.

The night before my instrument checkride my CFII put me through a simulated
checkride.

I suggest doing a simulated checkride, with a different, a week or so before
the real checkride. If you've done all your training with the same
instructor, and have not done any stage checks or similar, this is especially
valuable, for these reasons:

- independent review of your readiness by a third party
- time to work on any problem areas before the real checkride
- work out some of the nervousness by flying with someone else in a test
environment
- confidence builder for you.