Candle Burning Tips
Burning candles holds a golden rule 'trimming the candle wick' at least ¼ inch before burning it. An untrimmed wick can lead to black smoke and uneven candle burning. A wick can be cut using a regular scissor or nail trimmer. 
First burn of 4-5 hours is very important. Soy hold memory and if got uneven burn in first go, it might follow for rest of the candle, leading to tunneling of wax. . Let the wax cool down and then continue for another 4-5 hours burn. This procedure will not lead to mushrooming of the wick which can cause the candle to emit black smoke.
Never place a candle in direct contact with air. Even movement of people around it can cause the flame to flicker, which can also lead to uneven burning and black smoke.
While placing a group of candles together, always make sure to place them at a distance of 2-3 inches. This will help the candle to burn evenly and could possibly avoid the flame to heat the surroundings of other candles.