Ask questions: what sort of spell do you want? Who is it for? What will it do? Do you want the effects to be short term or long term? List everything you want your spell to be and plan accordingly. For example, if you want a fast-acting spell, you'd use tealight candles or 4 inch ones. If you want a long-term spell, you'd use pillar candles or other larger candles.
Do your research. Look around for the right supplies associated with what you want, for example, if you want a love spell you wouldn't use a black candle because black candles are designed for banishing energies. You would use red or pink instead. You should make sure you have the right things for the spell you want, otherwise the effects may be wrong.
I think it's most important for the spell to feel natural to you when you cast it. If I feel nervous or frightened during a spell, I tend to stop and not do it and instead revise the spell and the instructions: in my opinion, doing a spell should be like tying a shoelace or getting dressed: it should be a natural thing you do without feeling nauseous or nervous or unfamiliar. It's YOUR spell and so you should feel comfortable with it. Since you created it, you should have a connection with it to make it work better. If it doesn't feel right to you, maybe you should look back at it and try to fix it.