Don't waste your time with a spell you don't completely believe will work, because it won't.
Understand the spell and how it works before attempting; giving yourself a concrete mental image of the spell working, the factors going into, and the effects taking place it will indeed help you believe.
Not all spells require the exact ingredients listed; after all- the best spell, is a spell you created yourself. So if you tweak it a bit, and as long as you keep the supplies in corresponding properties, you'll be fine.
Meditate before casting a spell, it will calm you mental state and help you think clearly.