This file features a no-frills PowerPoint of 5 slides that shows examples of irregular preterit verbs inside the foot and "sandal" that encircles the third-person row. The sandal image is a rendering using the "free form" tool inside PowerPoint and can be copied and pasted over other examples and slides.
Included in the zipped file is a Word document on which I've digitally rendered the "sandal" over the 2X3 grid that will allow your students to practice, practice, practice.
There is a more comprehensive version of this info that serves as a complete lesson, with stories, illustrations, and animations. http://www.teacherspayteachers.com/Product/Spanish-2-irregular-preterit-teach-this-shoe-verb-like-trick-sandal-verbs-966203
But this one is a buck and will get you started!