Every time before you relight your candle, trim the wick to 1/4" to avoid soot and smoke. Keep the candle out of drafts. If soot starts to form on the side of the glass during burning, extinguish the candle, trim the wick and relight (or let it cool completely if it has been burning for 4 hours or more before relighting).
The very first time you light the candle, let the wax melt all the way to the edge of the container (let the entire top become liquid). Burn the candle for long periods of time (approximately 4 hours) every time to avoid tunneling.