Using the utility knife, cut the headrest material along the “X” lines. Do not cut the material along the other lines at this time.
This will leave you with an “X” cut as shown.
Pull the flaps up and cut the foam beneath the material to the proper depth. Cut all four sides of the foam.
Using your fingers, tear the foam out of the headrest leaving a recess where the shell will be inserted.
At this point you will need to install the harness up through the area into the recess. It may be helpful to follow one of the posts and tie wrap it to the post for restraint.