Burn leaves only when the wind speed is less than 10 mph, preferably less than 5 mph. Some wind actually feeds oxygen to your fire and quickly disperses smoke. Be certain the wind direction is away from your house, your neighbor's house or a busy road. Burn leaves when the relative humidity is low. The water content of hardwood leaves depends on the moisture in the air.
Dry leaves burn the best.
Rake at least a day or so after a rain, and burn them as you rake. Wet leaves smolder and cause obnoxious air pollution. Don't build the fire near flammable material, including other leaves. Rake an area around the flames. Set the fire on the downwind side of the pile.