Few things are as satisfying in life as getting even with an ex. Rather than do something illegal or untoward, you can use magic to exact some justice. To hex an ex is the ultimate in supernatural revenge.
Create a doll to represent your ex. Make the resemblance as close as possible, adding actual hair or fingernail clippings from the ex, if possible. Stab the doll repeatedly in the extremities in order to cause pain in your ex.
Use candle magic to bring about your mischief. Burn a candle while saying words that keep you focused on thoughts of harm to your ex. Picture your ex suffering. Repeat this ritual every day for a week, or for the best potency, perform the hex during the full moon.
Hang a pair of your ex's underwear over a rafter if you have one. Hit the underwear with sticks from a blackberry bush. This hex causes your ex to wake up in pain the next day.
Make an magical altar, and draw a pentagram on it. Put 13 black candles in the corners, sides and at the points of the pentagram. Stand at the alter and recant a hex (see Resources for hex ideas).
Picture the worst nightmare you can imagine, and repeat it in your mind several times. Picture your ex in detail. Picture sending the nightmare directly to the sleeping brain of your ex, with the dream ending in a night of terror and sleeplessness.
Write your ex's name on black paper. Burn it with a black candle while chanting the harm you want to happen.