Trim the wick!  Make sure to trim the wick to 1/4 inch before each burn.  If a flame gets too large it can cause black burn marks on the jar, smoke, and soot.   For ease with trimming the wick, purchase a wick trimmer.  

Please do not burn candles for more than 4 hours at a time.  The jar and wax get very hot and will burn off some fragrance oil, leaving you with a mildly scented candle.  My suggestion is to buy more than one candle if you want to burn a candle for more than 4 hours.  

Always burn a candle within sight. Keep away from children and pets. Keep away from flammable objects. Keep the candle free from any foreign materials. Only burn the candle on a level, fire-resistant surface.   Stop use of the candle when only 1/4 inch of wax remains.