It's safer to overdry than to underdry. Dry until firm, but not as crisp as a tortilla chip, not so dry that it shatters. When folded in half, jerky breaks. Jerky bends like a green willow. It won't snap clean like a dry stick.
Jerky is hard to cut with a regular knife.
A serrated knife or kitchen scissors work better. During the drying process, it beads up on the surface of the jerky. That's why it's important to cut off as much fat as possible when preparing meat and fish to make jerky. When making jerky from pork, it must be cooked and the fat removed before being dried. When you remove fat, you help eliminate more of the gamy taste.