Primarily all a spell is, is imposing one's will upon their reality or the reality of another. All the candles, incense etc are just focusing tools used primarily to stimulate the mind. Unless you're doing invocation or evocation, you generally do not need any tools. However the best advice I can give you for creating your own spells is to simply focus your own energy on what it is you're desiring, if you feel comfortable in using focusing tools then acquire them and use them according to your desire and process the spell through. If you want to remember it fully, write out what you used and what you did that made it work so you can have a personal book of shadows.
They wont increase the chances. If you allow doubt to enter your mind you are already failing. You must focus your desire and will into the spell. The only thing candles are used for would be a focusing tool. Something to help you focus.