What do teachers and readers ask for in a text? A clear way of consequent ideas that come to a logical conclusion. In other words, the best works are the ones that do not come to conclusions right away, but rather explain a set of causes and consequences to reach to them. It is not always so simple to present your ideas in a logical manner, so an outline can be useful in those respects.
Even when you seem to have very clear and abundant ideas, an outline will help you put them in the correct order and discriminate amongst them. What should I mention first? A general piece of advice is to rather start with a very general idea to conclude a very specific one, or the other way around. Respectively, these are usually called deductive and inductive approaches to arguments, and they are based on scientific methodological principles. This way, we make easier for our readers to follow our ideas and understand them.
Should I prioritize my arguments? Sometimes, we tend to think that the most we expand on what we say, the better. But it is actually the opposite: the fewer words to explain your ideas, the easier you make yourself understood. And this has to do not only with the order, but also to be humble enough as a writer and not attempt to explain everything. A solid argument is one that focuses on only one main idea. Outlines are usually made of topic sentences, that is, every first sentence of every paragraph you write. And every topic sentence should contain only one main idea, which is adequate to explore in one paragraph. Paragraphs with more than one main idea are often treated as messy and unclear by teachers. Remember that the goal is to help your audience understanding your text.
No one knows your topic better than you! which means that everybody else knows less than you do… Do not take any concept for granted. Introduce every word that may seem important to understand the rest of the text and do it as soon as you write it down for the first time. Do not wait until the tenth time you mention photosynthesis to explain what photosynthesis is! Introduce concepts by providing a definition. Nowadays, not only dictionaries provide us with meanings for words and, sometimes, a synonym may be more useful than giving the definition of the term, which is a solution to be found in modern language software tools. This kind of online applications can be used while you are working on your outline and may help you to get inspiration on how to introduce new concepts to your text. Just start writing and software tools like WriteBetter will give you ideas on how to continue with your paragraph! In this program you may find synonyms and antonyms for words and a dictionary which tells you the most common instances in which you can find a specific word!