First of all, this is a writing exercise, something to try in order to start your flow of words. Although you can write a Witch spell, you should not believe writing and then saying the words will make things happen as you want them to. Real magic comes from inside of yourself. I think using a spell (especially one you have written yourself, one you have put your heart into and given time to consider each word) can bring about your own magic through focusing you fully on what it is you want.
Having said that… write a Witchy spell. Think about what you want from the spell: love, hope, fame, fortune, a new car? Begin by calling on some outer force like the four directions, the elements, the moon, your Grandmothers. Pick something that suits you. Then write the words to say what you are asking for and give some information about the who, what, when, where, why and how. Keep it light, just have fun. Give it rhymes, rhythm and a pattern in the words themselves. Speak them aloud to see how it all sounds. Change anything that feels in the way. Have fun with it.
Whispers to the wind
Shouting into the fire
As I call out my ire
Dancing over the dirt
Walking in the puddle
As I feel in a muddle
Clear my thoughts
Help me to see
Me as I want to be.
How does mine sound?