A "modal" is a verb that indicates a need or a possibility. Because of this, modals also identify a "mood" in the verb. In the English language, modals and modals auxiliaries include : can, may, must, ought, shall, should, will, and would.
The Spanish version of a modal is very different because, syntactically speaking, the Romance roots of Spanish do not directly transfer from the Germanic roots of English. Some linguists even argue whether the term "modal" could be aptly used as an identifier in Spanish linguistics, since it seems to be mostly applicable to the morphology of English words. The evidence of this lays on the fact that only two verbs in the Spanish language, the verbs "poder" and "deber", are the equivalent of a much longer list of English modals.
For your presentation, the best thing to do FIRST is to establish this fact: that Romance and Germanic roots contrast tremendously, and that Spanish and English belong, respectively, to those two families of language. Believe it or not, a lot of people take this fact for granted or do not consider it at all.
Second, build a diagram, or any form of graphic organizer (I suggest a bubble map or a Frayer Model), where you present the English modals that are listed at the beginning of this answer.
Third, use a separate graphic organizer (just one showing two columns divided and use two separate colors *very important* and dedicate one column and specific color to PODER and another to DEBER. If you are not using a Smartboard, use either sticky notes or index cards to place under the correct translation.
Poder- To be able to: can, may, will
Deber: To be able to: should, would, must, shall, ought
It gets a bit confusing as you conjugate these verbs. For the sake of your presentation, unless you have high command of Spanish, try to stay away from conjugation. The reason is that, while some of these modals are repetitive in English, they transform a lot in Spanish.
An example using the word "deber", goes as follows:
I should- yo debo,
I should have: yo debi de/ debi haber
I should be: deberia ser
I should have been: debi haber sido
So, as you can see, it is best to stick to the history of the languges and the rules of syntax. Good luck!