There are many things you need to know before casting any kind of spell. First off, you must know enough about all spell casting before even thinking about starting. Go through this website and a few others to find out as much as you can. Any wrong move, whether intentional or not, can cause the spell to not work, or even backfire. Certain spells have to have a specific Moon Phase, specific candle colors or incense, or even a certain place/time during the spell casting. Always remember that before you cast a candle spell, you must have cleansed and blessed not only the candle, but your mind as well. More on this topic will be coming shortly!
Choose one or more candles in the color(s) appropriate to the goal of the spell (look up color correspondences).
Your candles must not have been used before (virgin), because used candles carry the energies of former use with them. But, if your spell needs to be done over several days, then you can use the same candles for that spell during those successive days.
Next you must charge the candle for your purpose. By charging it you bring your own energies and magickal desires into the candle. You can make a psychic link with the candle by charging it. You charge the candle(s) by:
1. Carving the name of your goal in it with a small knife. Do this in a concentrated space.
2. Annointing the candle, or rub it with oil while visualizing your spell. This is also called 'dressing' the candle. You can dress the candle with pure olive
oil (easier, but soemtimes less effective) or a suitable essential oil. In any magick that is 'calling' (attracting things, i.e. friends, etc.), you rub the oil from the top of the candle to the base. In 'banishing' magick (getting rid of things, i.e. bad habits, etc.), you rub the oil from the base of the candle to the top. Just use enough to rub on the whole candle, but not thick enough to make a mess. :)
Doing both the carving and the annointing is not necessarily more effective, but it may help in the concentration process. Some will find that they need both, some won't.
Now you may light the candle. Visualize your goal for as long as you can, or recitate what you want to happen for a time. Then put the candle out. Remember to use a snuffer; to blow it out may 'blow' away your desired result. Repeat this until you get results. Experienced Wiccans may find that they need less time, beginners may be doing it for a month. Developing your meditation skills to help with the visualisation is really the key. Then let the candle burn all the way out.
While casting a spell, always burn a white candle on your altar as well, as purification and protection against negative forces. This is your altar candle (some use 2 or more candles), which can be used for any ritual until it is burned away. Thus, in candle magick, there will always be at least two candles: one altar candle and one anointed ritual candle.