To reinstall the X clamp, first start by attaching two corners to the posts on the heat sink.

While holding the third arm of the X clamp down against the post attached to the heat sink, use a small flathead screwdriver to pry the X clamp away from the heat sink post.

As you pry, press down on the arm of the X clamp until you "walk" it down into the retaining groove cut into the post attached to the heat sink.