It is important to be confident, not just in the intention of the spell but in yourself when casting it. Magic doesn't work on a hope for the best basis, it works by the caster taking control of the process that will force the desired outcome. There is no room for curiosity or "fingers crossed" when performing a spell, you need an absolute belief in the fact that what you are doing will work. This can be hard to come by as it is a bit of a chicken and egg situation - it is hard to get the confidence in the outcome without having experienced successful casting but then successful casting is dependent on the certainty of knowing that what you are doing will work.
If I was writing a "how to" guide on casting a spell - these are the steps that I would suggest you follow:
1. Know precisely what it is you are trying to achieve and hold a visual image of your intended outcome in your mind throughout your casting.
2. Ensure you are aware not just of what you want your results to be but also the process by which your results will come about (You might want to increase your income for example, so you would work this back to establishing the fact that you need a promotion at work, identify the person who is likely to make the decision about promoting you and target the efforts of your work in this direction. This will have a much greater level of success than just sitting in front of a candle hoping for some money to turn up on your doorstep.)
3. (Slightly contradictory to point 2 but nobody ever said magic was simple) Be open to the fact that magic can manifest itself in different ways and expand your mind to allow for different paths and routes to actualising your outcome.
4. Understand what established traditions or correspondences you are bringing to a working and understand why you are using them. Don't just look things up in books, think why orange might be associated with luck.
5. Bring your own creativity to your work and again understand why you have chosen to cast in the way you have. Think about the types of magic and which will suit your purpose the best. Don't use fire for the sake or it for example, consider if a candle is necessary for your spell and what value it is adding for the working you want to perform. Try and bring a relevance to your outcome in every part of the spell you construct.
Success in magic is very much linked to a full understanding of what it is you are doing and why it is you are doing it.