Writing a conclusion is similar to writing a paragraph.
The first sentence in a conclusion is the topic sentence. It should be the most important sentence in your conclusion. Your first sentence in your conclusion must contain the main idea of your entire report.
The following sentence in your conclusion reiterates the main ideas from each paragraph. Your introduction 'introduced' your main ideas that were stated in your paragraph and your conclusion reiterates those main ideas. Essentially, you have (hopefully) given proof for your main ideas in your paragraphs, so, you can confidently state the ideas again as fact in your conclusion.
Your conclusion should be written in the following order:
Your first sentence must state the main idea of your entire piece of work.
The next few sentences should support your main idea by reiterating the main ideas from each paragraph.
The last sentence should be similar to the first sentence except that it should be in your own words. It should be your summary of the conclusion. For example, let's say you did a report on sharks. If your first sentence in the conclusion stated, "Sharks are efficient predators.” your last sentence can be something like, "Sharks are clearly the king of the ocean." This sentence supports that fact that sharks are efficient predators because it states their superiority as a species.
It is important to remember to organize your conclusion paragraph much like any other paragraph even if it is at the last part of your essay.