First, students will learn the etymology of the word congruent. That is, they'll see words with similar meaning and learn that congruent means same size and same shape. Then students will learn that corresponding sides and angles in congruent figures are themselves congruent. Here's a lesson on all that. Next, students will learn that figures are congruent if one can be mapped to the other via a series of rigid transformations. Here's a video on that. Finally, give your students this challenge, then this one.