My son is now 3 and a half years old and I felt like he was finally ready for me to teach him how to use scissors. He has great hand-eye coordination, motor skills and the strength to open and close the scissors.
This is an activity that requires your full attention, you can't just give your kid a pair of scissors and paper and walk away, you have to be there to supervise, so you don't end up with a kid with bangs or something worse than that.
I read several articles about how to teach kids to use scissors and I took a little bit of the information from each place to teach my kid. One thing I learned from my own experience: don't buy those cute animal scissors, they are too heavy! Well, if you buy them, put them away until your kid has the skills to use them. Buy a pair of kid size scissors with a blunt tip.
I drew different shape lines in regular white paper with colorful markers, showed him how to hold the scissors and kept telling him to open and close. Reminded him to never point the scissors at his face and to keep his fingers away, too. The only thing you want to cut is paper! ;)
He had fun, he was ready to try, he had asked me to use scissors before, so I guess that is a good indicator that your kid is ready.
*Some of my favorite articles were this one, this one and last but not least this one.