When I first took my basic one day intro class we had to sign a waiver first thing, before the class even started, absolving the teacher and the shop and even the folks at the college who organized the class of any and all responsibility, acknowledging that we knew blacksmithing carried inherent risks, etc, etc, etc. I'm sure there's a basic such waiver that could be modified easily available online. If I were just showing a couple of friends how to blacksmith, I probably wouldn't worry about it (that's just me, not saying you shouldn't worry about it!), but anything beyond that, yeah...a waiver would be involved.