Make absolutely sure you can trust them.
Granted, I did babysit almost every single one of my younger family members (both brothers, 4 cousins, and my half-aunt) when they were babies up til they were older, without older adults around to watch over me, but personally, I'm not that trusting. I know I was mostly responsible, but I've known plenty of other teenagers who just plain wouldn't be fit to babysit a rock. Hell, I've known adults like that.
I guess just show them how to do certain things, like diaper changing, feeding, etc., and then watch them do it to make sure they've got it down. If you can, call them occasionally while they're babysitting to check up on things. Make a list of what to do and when, like when lunch time is, or nap time. Set out or label the food you want the baby to eat (if not formula/stored breast milk). My relatives didn't do any of that for me, I had to figure it all out for myself, and I managed to do just fine, but I do remember wishing I had more of an idea of what to do.