Before you do anything, you must figure out what you wish for your spell or potion to do. This is vital for every part of spell crafting.
Energy is an essential part of magick as a whole. Learn to manipulate and channel your energy before starting.
Candles assist in spells through representation of the elements and providing certain energies represented through their colours. You will, of course, need to research the properties of the candles and what each colour represents.
If you wish you use herbs, research properties of herbs, and choose ones that match your purpose.
If you wish to use any other materials, feel free to. Spell crafting should feel natural, and it should represent you.
Words and Invoking Deities:
The wording you use should represent the spell's purpose. It should be a natural way to speak for you and should be easy to infuse energy with when you speak the words. When you are invoking a deity in your words, do not use any demanding words, and be respectful of that deity.