Candle Burning 101:
#1 Trim thy wick
The first time and every time you want to burn your candle, start by trimming the wick to between 1/8 and 1/4 inches long. You can use scissors, nail clippers, or a special wick trimmer but no matter what you do, always trim.
Why you ask? Trimmed wicks will give you a cleaner, brighter burn.
#2 Let the wax melt all the way across.
Once your candle is lit, don't blow it out until the top layer of wax has melted all the way across. This might take several hours — so don't set out to burn a candle at all unless you've got time to kill. You want to avoid the “terrible tunnel” that can occur if you fail to do a full burn.
It takes patience, but if you melt the wax all the way across every time you burn, the surface of the candle will stay flat and the sides of the jar will stay clean, all the way down until the candle is spent.
#3 Keep the flame away from moving air.
Do your best to keep your burning candle away from fans, air conditioners, open windows, or heavily trafficked areas where people walk back and forth a lot. Moving air can disturb the flame, leading to unsightly black marks on the glass.
Enjoy your candles!