How do I get the Apple AirTag in the plastic key tag holder?

Rotate the two halves in opposite directions away from each other to separate them.

Insert the AirTag into the bottom half (this is the half that has the 3 cushion pads) with the Stainless-Steel surface facing up.

Place the top half on top of the AirTag.

Rotate the two halves in opposite directions towards each other to lock them together.