On to the actual process of writing a spell.
The first thing you need to do is clearly define the purpose of the spell or your objective. A good beginners rule is to start with guidelines that are similar to those of making sigils.
Phrase it as though it’s presently true
Make it absolute
Make sure it’s descriptive and phrased in such a way that doesn’t leave a lot of loopholes for mishaps (i.e. I am ten pounds lighter could lead to you getting food poisoning and dropping ten pounds, not so fun)
These rules aren’t so hard and fast for spell writing as they are for sigils however. Play around with these guidelines and find what works for you! I prefer working more abstractly, working with the natural flow of events instead of shaping them overly much. If you work with a deity phrasing it as a request instead of an absolute may work a bit better.
For our purposes lets say our objective is to attract a lover. Now, we don’t have anyone specific in mind, we simply want to open up the channels that can allow love into our lives. The objective could be phrased a number of ways, “I am open to love and draw healthy relationships to my life” would work, as would “Love enters my life freely”.
Next it’s time to figure up what kind of spell you’d like to use. This is largely going to depend on what materials you have available to you and what kind of magic you’re comfortable working with. If you can’t cook you probably don’t want to be making food-based spells and if you don’t have jars that jar curse probably isn’t getting far. This is a fun place to get creative though. Have you ever wondered if you could use something in a spell? Now is the time to try it! (In all likelihood the answer is yes)