To start, give your child kid-size, blunt-tip scissors that open and close easily. Place his thumb through the round top hole, his index finger under the bottom oval hole, and his middle and ring fingers through that bottom hole. Have him pretend the blades are an alligator's mouth biting the paper as they open and shut -- and tell him to keep his other fingers out of the way.
Other fun ways to practice his new skill:
Slicing snakes. Let him roll play dough into long mama snakes and cut them into short baby ones.
Fringe benefits. Have him snip along construction-paper edges to create a funky place mat or crown.
Cool collage. Rip out pages from old magazines, give them to him to cut up, then help him glue the pieces onto paper.
Always supervise him while he uses scissors, and teach him to point the blades away from him at all times so he doesn't hurt himself (or give himself a haircut!).