Theres many ways to write spells, its kind of a 'whatever-works-for-you" kind of thing.
The way I do it is usually similiar to this:
I often start with a scrap piece of paper before I write the actual spell to write the information and plans of the spell down to make sure they fit up.
I start by figuring out the intent of the spell, What I want the spell to do for me and write it down.
Next, I look into the correspondences for when doing the spell. Lets say I'm writing a banishing spell to repel negativity:
The best day for banishing is Saturday, Moon phase is Waning.
Next I'd look into the supplies, like candles, Incense/herbs, crystals, etc. So for banishing I'd use a black candle, Dragon's blood incense, and Black Obsidian.
Now I'd write the steps of the spell, the imagery and the casting instructions. Then write the words of the spell, using rhymes if I like.
Now I take all the information I wrote down and make sure everything matches up and fits together. Using correspondences makes the spell more powerful, then everything is in strong harmony with the intent. Then I form it into a spell sheet, looking similar to this:
Name Of Spell:
Type Of Spell:
Purpose/Intent of Spell:
Time To Cast:
Name Of Spell: Banish Negativity
Type Of Spell: Banishing
Purpose/Intent of Spell: To Banish Negativity and bad energies from body, mind and soul
Time To Cast: Saturday, On a Waning Moon
-Dragon's Blood Incense
This is one way I use, I hope it works well for you. Theres many ways, so experiment with it.