I just had a buddy fish sit over thanksgiving, he didn't "listen" to my instructions, I came home to a reeking tank full of waterlogged rotting pellets, I was ticked. I suggest demonstrating everything to the fish sitter, then have him do it with you there the first time and also write everything down.
I even make sure to have (my normal fish sitter) know how to check temp, make sure the filters are working, what to do in an emergency, etc. )
It seems to be my experience that if you just tell someone what to do and then leave for your trip, they get there to feed and forget everything you told them!
I have found it best to try to get the same person to always do it because they learn more and more each time. Leave you cell # by the fish tank!!