Getting started:
Your first burn: trim your wick to 0.5cm before burning. This will help prevent soot and extend the life of the candle. The initial burn must last till the wax pool meets the edge of the vessel - this helps prevent tunnelling.
Wick trimmers:
You can use scissors or a specialised wick trimmer! Trimmed wicks will give you a cleaner, brighter burn. Untrimmed wicks are a lot more likely to take on a weird mushroom-esque shape that dulls and obscures the flame. Beyond that, trimmed wicks keep your candles clean so you can avoid burnt rings around the jar/vessel, and soot fall. Trim your wick as needed.
Burn your candle 3 hours at a time:
This is so the melted wax created is not too excessive, and that the glass does not get too hot. Burning for 3 hours at a time also prevents tunnelling – as with too much wax the wick may start to lean, as it will not be stabilized enough in the wax to stay straight.
Candle snuffers:
Using a candle snuffer minimizes smoke plume (this works by depriving the candle of oxygen), and also stops particles from the wick been blown into the wax pool beneath the wick. A glass lid can achieve this result as well, but make sure the lid/underside is burn proof first!
Dispose of candle when 1cm of wax is remaining in the vessel:
Excessive heat collecting in the base of the vessel from the wax along with potentially some foreign particles from the wick or air may damage the integrity of the vessel or create burn marks at the base of the vessel. Also with this heat collecting, the base of the vessel will become very hot and may damage surfaces and be harder to handle.