Sometimes the best way to get started on a paper is to imagine–i.e., draft–your introduction. Don’t overthink this. And don’t spend too much time worrying and working on it. Just do it. Spend a few minutes at the end of the day deciding what you will say (construct the key sentence) and 27 minutes the next day writing what you know about it (composing the paragraph).
Then write a paragraph that asserts your conclusion. You might simply remove the words “This paper shows that” from the key sentence of last paragraph of the introduction, which should leave you with a declarative sentence that states your result. Draw the substance of your paragraph from your analysis. That is, base your assertion on your data. Write the strongest statement of your conclusion you are capable of. Imagine the friendliest and most knowledgeable reader you can. This is how you would say it to yourself or your co-author.