Read through your draft and split it up into sections by topic. Try to come up with a good, short header for each section. Writing the introduction. Now go back and look at your initial topic statement. This is probably now somewhat outdated, but it may provide a basis for your introduction, which you have not yet written.
Introductions are fundamentally problem statements.
Since you have already written the body of the article, and you know what you are going to say in it, the introduction should be fairly easy to do. It is very difficult to write an introduction for a paper that rambles from topic to topic. Revising the initial draft. Now rewrite your initial draft, reorganizing and rewriting to make the writing smoother and the explanations and transitions as clear as possible. Where you have particularly abrupt changes of topic, you should try rearranging the text or adding a missing section header before you try smoothing over the gap with a transition sentence.