Introductions Are the Last Part of the Speech to Prepare
While this may seem both counter-intuitive and somewhat strange, you really do want to leave the development of the introduction for the last part of your speech preparation. Think of it this way: Until you know what you are going to say in your speech, how can you introduce your speech?
The introduction is prepared last because you want to make sure that the body of the speech drives the introduction, not the other way around. The body of the speech contains most of your content, your arguments, your evidence, and your source material: The introduction sets up the body, but it should not overwhelm the body of the speech, nor should it dictate the content or structure of the speech.
Once you have the body of the speech complete, then you consider the introduction. With the body of the speech complete, it is relatively simple to complete two of the four functions of the introduction. You already know the purpose of the speech, so now you need to put it in a one-sentence statement. And you already know the structure and main points of the speech, so you can put that structure into the preview.
With the structural functions of the introduction complete, you can carefully choose and craft the type of introduction you wish to use, and concentrate on making sure that the introduction also fulfills the other two necessary functions: Gaining the attention and interest of the audience, and gaining the goodwill of the audience. The introduction is last.