Extemporaneous delivery is probably worth as much time as you can afford to spend on the project. By extemporaneous we mean you know what you are going to say and you have the content well in mind. You have organized the data appropriately for your listeners and when the time comes for you to speak, you choose your words while you are speaking.
Aristotle thought that between content and delivery, delivery was not nearly as important. Demosthenes on the other hand felt that delivery is critical. Personally I think that excellent content excellently delivered should be the goal. In life, you will most often be asked to speak on things about which you are well informed. Your biggest problem will be matching the audience to you and your topic. That will guide you in what to include, while you are narrowing your topic. Then the order in which you have been most successful talking to others about the topic.
Task: establish a central idea and type it across a blank page in your word processor, and save it to your desktop. Whenever you think of something that should be included in your presentation, open the document and add it. As potential main ideas occur to you, add them. As supportive material occurs to you, add them under the appropriate main idea. Out of this will grow your fairly detailed outline.
Practice: examine your materials, adjust them to fit you, your audience and the occasion. Let the rough outline sit for a time and then go over your materials and set a final version of the outline. Use that outline to build the visual materials (PowerPoint, Keynote, etc.) you will need to clearly make your points. Remember, the more ways you can simultaneously say a thing the easier it will be for your audience to understand and retain what you are saying. Remember, in the "real" world you will be asked to say to a large group things you have already been saying to individuals and small groups. You have practiced your speech. More practice won't hurt and could help.
Delivery: get to your location early, setup and test everything. Once you stand up and begin realize that there is nothing more you can do to prepare. Your only concern now is that the audience will understand and be able to recall and use what you are saying. It isn't important that you are perfect in appearance or delivery. . .only that the audience understands, can recall and use your content. You are at this point more like a missionary than anything else: only concerned about the audience.
Afterwards: what kinds of questions are asked? What might you have done to make clearer what you were trying to say. Make notes and then make repairs, because it is likely that you'll be asked to do this again fairly soon.
Don't forget to make available leave behinds. Be certain that your contact information is clearly part of the leave behind.