Well first you should figure out the intent of the spell, what you want it to do and if you are ready or even want that result.
Next, do some research, find the supplies that you will need and make sure they are in balance with what you want. Using things like Herbs, Candles, Stones, symbols, charms, etc help with that but you don't necessarily need supplies.
The most important thing is the words, in my eyes at least. You should be direct with what you're writing, it should make sense and be honest and descriptive to what you want. you can use rhymes if you want, or just words that you feel you need to say.
You can also look around and find some invocations or spells to refer to while writing the spell.
When you cast the spell, make sure you're in a good mindframe. Do some meditation before hand to center yourself, go in a private place where you can be alone and just relax your mind and body, if you don't do that, chances are you're gonna have trouble making the spell come to life.
While doing it, envision what you want happening, think about that very, very clearly and focus only on that.
Give the spell time to work, it might not happen over night or as soon as you cast the spell. It can take a while so just be patient, if it is supposed to happen it will.