Relax a little! Easier said than done though, I know.
If you make a mistake and the owner for some odd reason does not know the definition of "mistake", then the lease arrangement probably isn't worth your time. Just brush yourself off and keep looking for a better match.
I was always told being too nervous, tense, and stressed travels down the lead rope (or reins). I think if you take a deep breath, keep in mind what you know, and look towards the bright side you will do fine.