How to prevent candle tunnelling
The first time you light the candle, don't trim the wick. This is the ONLY time I recommend NOT trimming the wick before you light a candle. Depending on the wax and size of the vessel, you may need a flame that burns large and hot enough to melt the wax all the way to the edge. If you trim the wick too short, the flame might be too small to melt all the wax. This is particularly true of wide candles. If, after you light the wick, there is any smoke at all, then extinguish the flame and trim the wick slightly. You never want any soot or smoke at any time - ever.
Let the candle burn long enough to get the entire top liquid wax. This takes about an hour per inch in diameter of the vessel. So, if your candle is 3" wide, let it burn for at least 3 hours. If it doesn't melt all the way to the edge don't panic, but get as close as you can. Sometimes the last 1/4" will burn as the candle goes down. It won't burn evenly across the top, but it will burn completely.