You live and you learn..I've made the mistake of having a neighbor watch/feed my fish once - and I spent way more the 20 minutes training..I was gone for two weeks. I was left with a mess when I returned. What they thought was normal, I viewed as a disaster. Either you get a friend (that knows what they're doing) or do what I did and buy a feeder.
I bought a Eheim everyday fish feeder a month ago for about $40. Put three types of food (flake/pellet/crumb mix - floating/sinking, etc) set the portion/feeding times and tested it out for the 2 week period I was leaving for - ahead of time. Worked like a charm in testing AND when we were gone...no over feeding and aside from the fish being a little skittish when we came back, It was like we never left.