You probably end up reading the paper several times. While you want to be critical of the material presented in the papers, you want to give a fair presentation of its contents. Attempt to be an advocate of the authors. If portions of the paper do not come to you, feel free to contact the lead author. They will be thrilled about your inquiry, because there is nothing like knowing that people actually read your papers. More likely than not, they will be more than happy to explain things to you and provide you with additional materials. So please start on your papers early, so as to get the chance to contact authors. Hey, you may decide to go to grad-school to study with them!
For your presentations, feel free to make slides of figures in the paper, from the authors web-sites, or to demonstrate software that you may have acquired from them. Don't forget to leave ample time for discussion, which may take place during your presentation or afterwards. If you have questions about your presentation, please get in touch with me.
Finally, provide me with a draft of your presentation at least 24 hours before you give it.