I've heard that visualizing everything works, but I haven't tried it. Some people told me that a drawing of a candle or a virtual one also works. Using nothing at all is okay too; it's all about visualizing, centring, grounding, energy, etc. Good luck to you and your friend.
Substituition in Magic is all well and good but make sure you alter the recipe in question to fit your purpose instead of just chopping something out and replacing it without any reason why. For Candles virtual are not recomended. Candle Spells sometimes ask for the remains of the Candle to be saved and used in a Satchet and thus a Virtual one is inappropriate.
Also, when making spells be sure to really think about what you can get easily, keep it simple. The more complex the spell the harder it will be to do.
Not all Spells call for Crystals and Candles, some spells just require paper, pen and fire. Others just words, some just actions and thought.