You need an aromatase inhibitor like arimidex or aromasin, and ideally you should cut to 15% or lower before starting. Fat tissue has higher levels of aromatase so you're more likely to get estrogenic side effects the higher your bodyfat.
Diet should essentially be the same as if you were cutting natty. Very high protein intake, eat below your maintenance number of calories. Since you're on gear you can reduce your calories even further without losing as much muscle, but essentially it will be the same. Switching all your carb sources to more "complex" ones will help, though calories will make the largest difference. You can get below maintenance either by upping the number of calories you burn, or by eating less that part's up to you.