Read through a given paragraph twice, if time permits. People tend to skim or rush through words when reading something. You can overcome this habit by paying attention to each word. For younger readers, using a finger to point along the line or a piece of paper to cover up the lines below what is being read is helpful.
Underline the sentence that summarizes the most in the paragraph. In most nonfiction writing, the topic is toward the beginning of the paragraph. It may even be the first sentence. If you have read the paragraph twice, you should have a handle on the details of the subject matter. Challenge yourself to edit the paragraph down to the sentence that has the most meaning or provides an overall synopsis of the paragraph. This is the topic.
Highlight single words or phrases to emphasize the main ideas. These are generally the major points brought up about each topic. After the topic sentence, authors tend to use explanation to state their point. The main ideas are short sentences that describe the topic while emphasizing something new. Again, use your insight about the topic to edit to just a handful of phrases taken from the sentences within the paragraph. Look for supporting points that are defended by reasoning or logic. The main idea will be at the beginning of each defense.
You can practice these skills by reading a variety of articles and papers. The more you read, the easier it will be for you to apply this technique. For some practice, read the following paragraph twice: Giraffes are a unique species of exotic animal for several reasons. They have patterns of brown and yellow. Their colors make them different from other animals, but other attributes set them farther apart. Giraffes have tall necks that help them reach tall trees. Some may find their necks to be aesthetically pleasing.
In this example, the topic is stated in the very first sentence since it provides a basic overview of the entire paragraph. The first main idea is the 'patterns of brown and yellow'. This phrase would be highlighted according to the technique described. The next main idea is that giraffes have tall necks. Each of these main ideas is somehow elaborated by the succeeding sentence. This is an indicator of each being a main idea.