Research basic spell casting skills, and look up different tools for spell casting and how they are used. The most important thing about spell casting is keeping your intention in mind. Some people use methods such as sigils, candle magick, herbs, stones, etc. in order to perform a spell. Know that the tools you use aren't nearly as important as focusing on what you want to happen. You are the only tool you need. Look at some other spells people have written if you need examples. After you have researched a lot of these things, then you are ready to write your own spells, which is personally, my favorite way to cast spells.