The Witches' foot is a set of four principles that are very important to successful magic: To Know, To Will, To Dare, and To Keep Silent.
To Know
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Do research on the kind of spell you're trying to do. For example, if you like to use candles in magic, read about what other people have done with candle magic. What steps do they go through? You don't have to make your spell exactly like someone else's, but if you notice everyone else dressing their candles with oil while concentrating on their purpose (for example), it may be something that you want to try as well.
Also learn about the magical correspondences for your tradition and use them in your spells. For example, if you were doing a love spell and you were a fan of the Norse pantheon, you might want to emphasize certain runes, invoke the goddess Freya, and do the spell on Friday (Freya's Day). There are also correspondences in most traditions for colors, plants, animals, times of day, seasons, and phases of the Moon, among other things. The more of these you can incorporate into your work, the more focused your energy will become.
Go over the spell in your mind a few times before you actually do it. Know what step comes next. If possible, memorize your lines.
To Will
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Now that the spell is focused, YOU must be focused as well. If you throw a spell together at the last minute and do it half-heartedly while watching The X-Files, no matter how skilled you are, you will get less "bang" from your magic than you would have if you prepared well and really concentrated on what you were doing. A regular practice of meditation will teach you to focus your mind, which will in turn fuel your magic much more effectively.
You must have confidence in your spell for it to work. You must believe that you have done real magic and that real magic works. Your spell feeds off of your thoughts and energy, and if you don't feed it well, it will die.
To Dare
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It's not only your conscious mind that has to believe in the spell, either. Any ambiguous thoughts you have lurking about will compromise your magic.
On the top of a sheet of paper, write what the spell is basically for: "A Job." for example. Write down every thought that comes into your head about "A Job." When you run out of ideas, read over your list. How many negatives are in it? These are thoughts that will interfere with your spell unless they are dealt with. For example, if I do a spell for "A Job." and I discover that deep down I hate most jobs and would rather not have one, for my spell to work I either have to change my goal or deal with my attitude problem.
The other part of daring is infusing your spell with some emotion. Emotion is energy. "Gosh, I wish he would leave me alone" will not give much punch to a hex. (I don't recommend hexes anyway, but this is an example of using emotion for energy.) "LEAVE ME THE HELL ALONE!!!" will do better. Feel something as you're doing the spell. Give a flying fig how it turns out. If you don't care, why should the Gods? (And for that matter, if you don't care, why are you doing the spell in the first place??)
To Keep Silent
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Now that you've done your spell, shut up about it. There are two reasons for this. One is that the energy you spend dwelling on it is energy that isn't going to the spell itself. It's more efficient to just think about something else and let your spell do its work. The other reason is that if you talk about your spell to someone who doesn't believe in magic or who doesn't want your spell to work for some reason, that person can undermine your confidence, which will weaken the spell.