When writing, or casting a spell, the first thing to consider is whether you truly need magic to achieve your desired goal(s). Take a moment to think about a few things. Can your goal be reached by normal means? Can you personally work toward achieving this goal without the use of magic? Is your goal unrealistic or something that you could not achieve by any normal means whatsoever? If so then a spell may and usually is not necessary.
If you do decide to use a spell and write your own, I have provided a brief guide for you to use when writing your spell. I have provided a brief explanation for each item listed. You can use it as a tool to help give you ideas on how to go about writing your own spell(s). Keep in mind that you don't have to include everything in this list when creating your own spells, this is just an example of things you may wish to include.
Intent: Your intent is your focus or your desired outcome. Before writing or casting a spell you should meditate on your goal to help you better focus on it during spell creation and casting.
Lunar Phase: Lunar phases correspond to specific magical aspects and can help strengthen spells and rituals if performed as this specific time.
Day: Days of the week correspond to specific magical aspects and can help strengthen spells and rituals if performed on a specific day.
Symbolism: You may wish to use certain magical symbols or simply symbols in general to help strengthen your focus on the spell casting and creation. For example, if you are creating an herbal sachet you may draw the symbol of a shield on it to further imbue and charge it with protective energy.
Herbs and Plants: In some spells you may wish to use herbs or other plants in the casting of your spell.
Herb or Plant Correspondences: Magical association of herb or plant.
Candle Colors: If you use a candle you may wish to have its color associated with the outcome of the spell.
Chants: Any words or phrases that you feel you should say during your spell casting to help strengthen your spell and the chances of its success.
Results: I would suggest keeping track of your successes and failures when creating and casting spells to help you grow and gain experience on how to better your chances of success.
You can use whatever you like. As I stated above, the information provided was just to get you started and give you some ideas. You can do whatever you like if you feel it will help you better create and cast your spells.
One does not have to be fancy with spells. It don't matter if it rhymes. It doesn't matter how long it is. It doesn't matter the time it takes.
What matters is whats going on in your head once your doing the spell. What matters is the relations between you and the deity (If mentioning any). What matters is that you clear your mind, concentrate energy in your spell. Believe in your spell, live by your spell. Fell the emotions between you and your spell. Visualize and meditate on that spell. This is what really matters. Your spell can be three sentences long, just do what matters!