Sunburn results from putting plants in sun before they are "hardened" to it, they need time to develop protection to their cells, the same as you do. By starting slowly and gradually lengthening the time the plant is in the sun, you will allow the plant to toughen up enough to take the sun. Interupted sun by a screen of other plants so only broken sun reaches them is another method. Also be sure you use a watering tray, and keep the water level high enough so the plant never dries out. Depending on the stress, the plant should recover in a couple of weeks or so.